Method and apparatus for electric treatment of substrates

ABSTRACT

Methods and apparatus for hydrodischarging and hydrocharging substrates and articles to produce enhanced ability to avoid attraction of contaminants or improved capability of removing contaminants from fluids are disclosed. In another form the method involves removal of electric charges or neutralization of charge on or within substrates. Also disclosed are methods of making using electret substrates and articles for removing particulates and mists from fluid streams.

CROSS-REFERENCE TO RELATED APPLICATIONS, IF ANY

This application claims the benefit under 35 U.S.C. §119(e) ofco-pending U.S. Provisional Patent Application Ser. No. 60/680,270,filed May 12, 2005, which is hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX, IF ANY

Not applicable.

BACKGROUND

1. Field

The invention concerns improved methods and apparatus forhydrodischarging and hydrocharging substrates and articles to produceenhanced ability to avoid attraction of contaminants or improvedcapability to removing contaminants from fluids. The field of theinvention relates to electret substrates and of filtration media.Additionally, the inventions relate to controlling electrostatic chargeon substrates and cleaning substrates. The invention concerns themodification of electrical charge properties of substrate. The inventionconcerns in one form electret enhanced filter media made of substrate,and fibers such as blown microfibers. The invention concerns improvedmethods of making electret substrates and articles for removingparticulates and mists from gas streams. In another form the inventionconcerns the removal of electric charges or neutralization of charge onor within substrates.

2. Background Information

The addition of electric charge to a substrate is quite useful. It isknown that substrates including polymer materials may besemi-permanently electrically charged, or for brevity, charged. Whencharged, such polymers are known as “electrets”. Electrets havesignificant commercial value. For instance, the electric field producedby the electret can be used to attract other materials, such as dustparticles. This attractive or “inductive” property exhibited byelectrets substrates enables filters to be constructed having theability to capture sub-micron particles when the pore sizes are manytimes larger. The removal of electric charge from a substrate is alsoquite useful. Often it is required in the manufacture of substrateintermediates for many products to prevent dust contamination. Objects,including humans, very often acquire a sizable electrostatic chargewhich may have a magnitude of several thousand volts or more. Chargingof non-conductive objects may be caused in many ways includingfrictional contact. Induction and discharges from other objects mayimpart charge to ungrounded conductors. Sizable charge accumulations canbe highly undesirable for a number of reasons in the processing ofdielectric materials and semiconductors. Sudden discharges even when notharmful are distinctly unpleasant to people. Electrostatic charge canalso interfere with the operation of electrical devices includingintegrated circuits. Very important is that charge also attractscontamination.

The substrates include for example plastic films, paper, nonwovens,fabric materials, dielectric materials, and nonconductive materials.These are commonly used as the base of construction for a wide varietyof greatly differing products. Some examples include photosensitizedfilm, photographic print paper, magnetic recording tapes, adhesivetapes, pressure-sensitive paper, packaging materials, signage, filters,wrapping materials, electronic substrates, optical films, and cleaningproducts.

Prior art often requires the use of complex or hazardous processes forneutralization of static charges on a web or substrate, for theelectrostatic modification of substrates, and for the production ofcharge species on or in the substrate. All of the known methods havevarious limitations and problems which restrict their utility oreconomics. These are discussed delineated in the following review.

Hydrocharging for the Production of Electrets

Electrets are dielectric objects that exhibit a lasting electric chargeor a charge that is at least quasi-permanent. The charged nature of theelectret enhances the electret's ability to attract and retain aerosolparticles, and contaminants such as dust, dirt, and fibers that arepresent in fluids. Electrets have been found to be useful in a varietyof applications including air filters, furnace filters, respiratoryfilters, face masks, and electro-acoustic devices, headphones, andelectrostatic recorders. Commonly, nonwoven or fabric substrates areused in filtration. Electrets are especially useful for collectingmicron and submicron size particles or aerosols on or within media whosepores or void spaces are much larger.

Nonwoven fibrous filter webs have been made from polyolefins usingmelt-blowing apparatus of the type described in Wente, Van A., SuperfineThermoplastic Fibers, Industrial and Engineering Chemistry, v. 48, n. 8,pp.-1342-1346 (August 1956). Such melt-blown microfiber webs are inwidespread use for filtering contaminants, e.g., as face masks, furnacefilters, and respirators. Melt-blown microfibers are commonly referredto as blown microfibers.

It is known that the filtration qualities of a blown microfiber web canbe improved by a factor of two or more by making it an electretsubstrate. In one method the melt-blown fibers are bombarded withelectrically charged particles such as electrons or ions as they issuefrom the die orifices. Similarly, the web can be made an electret byexposure to an electric corona after it is collected. While blownpolypropylene microfibers are especially useful, other polymers may alsobe used including, for example, polyolefins, polycarbonates andpolyhalocarbons. Most commercially useful are those materials that haveappropriate volume resistivities under expected environmentalconditions.

Filters for removing particulate contaminants from air and fluids arealso made from other types of media. Examples include spunbond nonwovenmedia, woven fabric media, structured films, porous films andfibrillated films. U.S. Pat. RE32,171 to van Turnhout teaches thatelectret filtration enhancement can be provided by electrostaticallycharging a film before it is fibrillated. However, the method uses highvoltage charging which adds manufacturing expense and electricalhazards.

Hydrocharging is generally considered a process for preparing electretitems and substrates without the use of high voltages, but it presentlyhas deficiencies. It employs a liquid as a charging agent. Angadjivandet al. in U.S. Pat. No. 5,496,507 teach impinging water upon a nonwovenweb with jets or droplets, then drying the web to create an electretmedia. While the technique has been described by some as a method oftriboelectic charging, the details of the electrification process arenot fully explained. Hereinafter “hydrocharging” refers to thecontacting of a substrate with a liquid to create an electret. While theAngadjivand et al. hydrocharging process develops filtration enhancingproperties, the degree of treatment is deficient and pre-charging theweb by corona charging prior to hydrotreatment is necessary for the bestresults. This teaching still requires the capital investment and theoperating cost of corona generating devices along with their highvoltages. More effective processes are sought.

Further improvements in hydrocharging are taught by Eitzman et al. inU.S. Pat. Nos. 6,406,657 and 6,824,718. They teach the multiple steps ofwetting with a wetting liquid, followed by saturation with an aqueouspolar liquid, followed by drying. Wetting liquids with surface tensionsbelow the surface energy of the fiber are taught for the wetting step.But even so, air is trapped in the web and the use of mechanical meansto help in the removal of trapped gas is taught. The use of an aqueouspolar saturation liquid with a surface tension higher than andpreferably 10 dynes per centimeter higher than the surface energy of thefibrous web is required after the wetting step. Careful formulation andcontrol of both wetting liquid and the aqueous polar saturation liquidformulations are required for the process to function. The organicsolvent isopropyl alcohol is the wetting agent of the examples. As such,the process uses an expensive and hazardous chemical to achieve results.

Eitzman et al. in U.S. Pat. No. 6,454,986 teach the use of flammablepolar organic solvents by themselves to create electret media. Thisstill has the disadvantages of high costs, and explosion and firehazards associated with the solvent. Surface tensions of 10 dynes percentimeter higher than the surface energy of the fibrous web arepreferred. Wetting is not complete. A partial remedy of this problemusing various mechanical agitation means are taught to aid wetting.These add cost and complexity and have limited success.

Horiguchi and Takeda in USPTO Publication Number 20040023577 expound amethod of hydrocharging by sucking water through a fibrous substratefollowed by drying. The efficiency of the process is deficient. Theynote the electret quality is improved by repeating the suction processmultiple times before drying. The teaching recommends use of a wettingsolvent in the water. Elimination of repeating steps is desired toreduce process cost and complexity. Elimination of cost and hazard ofthe wetting solvent would be a cost saving. Wetting problems are stillpresent even with the wetting solvent.

The process of depositing liquid from a vapor onto a dielectric articleprior to drying is taught in U.S. Pat. No. 6,743,464 to Insley et al.Here, the method utilizes the complicated process of first creating acontrolled environment using a closed vessel containing a liquid and agas phase, and second, manipulating a thermodynamic state function suchas pressure to cause molecules of the liquid component present in thegas to condense as liquid drops upon the article. This condensed liquidon the article is then dried. The gas phase contains noncondensible airdiluting the molecules of the liquid species present in the gas phase.The complexity of this method is a disadvantage. Additionally,condensation must take place, no means of removing trapped air in thearticle is provided and no method of treatment of a continuous web istaught. The method does not teach a means of complete and total wettingof substantially all surfaces in pits, voids, pores and internal spacesof the substrate.

Improved and simplified hydrocharging methods are needed that do not usecostly or hazardous materials, do not require high voltage electricalauxiliary treatment and do not require special chemical formulations.Methods that do not use polluting chemicals are sought. Methods that actwith the improved efficiency are needed. Methods that produce improvedcontacting of all surfaces in pits, voids, pores and internal spaces ofthe substrate and improved methods for treating continuous webs areneeded. Methods that remove greater amounts of air from the surfaces andpores of the media would result in the more complete treatment of allthe potentially functional surfaces of the substrates.

Electret Surface Contamination

An additional problem for electret media is contamination. Oilycontamination from a gas stream being filtered is noted as being highlydetrimental to the efficacy of filtration products in U.S. Pat. Nos.5,411,576 and 6,802,315. Much effort has been devoted to producing oilmist tolerant electret media. However, other sources of contaminationhave been unrecognized. One is the problem of surface contamination ofthe electret media during manufacturing, which has not been recognizedand addressed.

The processes necessary for the forming of the substrates, and theprocessing and handling of the substrates may contaminate the activesurfaces. In the production of fibers and films, high temperatures areused. The materials are extruded in a molten state. Thin films, veryfine fibers, and especially melt blown and spun bond fibers are extrudedfrom melts and are most easily produced when the melt viscosity is aslow as possible. Low melt viscosity is achieved at extremely hightemperatures. Often these temperatures exceed the thermal stability ofthe materials extruded. At high processing temperatures thermaldegradation forms oil-like low molecular weight contaminants. Theoil-like liquid degradation products produced commonly cover thesurfaces of fume hoods over these melt process lines. Smoke and fumesare often observed rising from molten polymers being extruded, milled,or melt spun. These then may condense on, or be adsorbed by thefunctional surfaces of the electret substrates or precursor substratesproduced.

The process equipment used for transporting, forming, collecting andextruding substrate materials uses hydraulic and lubricating oils alongwith other liquids which are electret contaminating species. These willoften contaminate filter media. Oil and decomposition contaminants tendto spread and cover the active surfaces of many common polymers used forfilters. This is especially true of the polyolefin polymers. Suchcontamination can diminish either the initial or long term performanceof electrets.

Here it has been found that any liquids that spread on the substrate orsubstrate functional surfaces are detrimental. Still other harmfulcontaminants include species that modify the wetting characteristics ofliquid, collected on the electret and cause liquid to spread on thesubstrate surfaces.

The problem of counteracting contamination from the forming methodsremains unsolved and generally unrecognized.

Electret Substrates for Filtering Liquid Mists

The filtration properties of nonwoven and fabric polymeric fibrous webscan be improved by transforming the web into an electret. Electrets areeffective in enhancing initial liquid aerosol particle capture infilters. But with time or aging, liquid aerosols tend to cause electretfilters to lose their enhanced filtering efficiency. This subsectiondeals with the art of preparing improved aerosol filter media.

Numerous methods have been developed to compensate for loss of filteringefficiency with time or aging in the presence of mists. One methodincludes increasing the amount of the nonwoven polymeric web in theelectret filter by adding layers of web or increasing the thickness ofthe electret filter. The additional web, however, increases thebreathing resistance of the filter, adds weight and bulk to the filter,and increases the cost of the filter. Another method for improving anelectret filter's resistance to oily aerosols includes forming theelectret filter from resins that include melt processable fluorochemicaladditives, such as fluorochemical oxazolidinones, fluorochemicalpiperazines, and perfluorinated alkanes.

A method of improving the performance of an electret is taught by Jones,et al. in U.S. Pat. No. 5,472,482. It teaches placing a performanceenhancing fluorochemical additive into the polymer melt, extruding theblend in the form of a microfiber web, and then charging the web. Theseadditives are referred to as “charge additives.” U.S. Pat. No. 5,645,627also teaches the use of charge additives. The charge additives canincrease the level of charge on the electret and can improve thefiltering performance of the electret. The charge additives have beenfound by experimentation. Charge additives within the mass of polymermust be melt processable, i.e., suffer substantially no degradationunder the melt processing conditions used to form microfibers of anonwoven web or the fibers and films of electret substrates. This limitspossible candidate additives.

The improved performance in the additive patents is only demonstratedwith one, and only one, aerosol liquid mist. This unduly limits usefulcandidate additives. Tests are only made with dioctylphthalate (DOP) inair at standard conditions. The test is hereinafter referred to as the“DOP challenge”. No information is provided for filtering other liquidaerosols in other gases and at other conditions.

In U.S. Pat. No. 5,935,303, an improved filter is taught which uses aresinous material containing a perfluoroalkyl acrylate adhering to thefibrous substrate in a filter. This improvement is again only testedagainst a DOP challenge. No information is provided for filtering otheraerosols and mists.

In U.S. Pat. No. 6,213,122, a method of making an electret with improvedDOP filtering performance by including the step surface fluorination istaught. Fluorination is a costly and sometimes a very hazardous step.Again, the electret is only tested against the DOP challenge.

In U.S. Pat. No. 6,238,466, an electret article with improved oily mistperformance is disclosed where the formulation includes a chargeadditive and passes a thermally stimulated discharge current (TSDC)test. However, again the electret is only tested against the DOPchallenge.

U.S. Pat. No. 6,214,094 teaches electret articles using charge additivesto produce improved DOP challenge performance. Here too, the electret isonly tested against the DOP challenge.

U.S. Pat. No. 6,802,315 discloses electret articles using vaporcondensed coatings with fluorine contents that give improved results.Here too, the electret is only tested against the DOP challenge.

U.S. Pat. No. 6,237,595 teaches electret DOP filtering performance maybe predicted by measuring extractable hydrocarbons.

U.S. Pat. No. 6,261,342 teaches electret DOP filtering performance maybe predicted by a thermally stimulated discharge current (TSDC)spectrum.

USPTO Publication Number 20030054716 teaches treating porous substrateswith a solvent composition which includes a charge additive forenhancing performance for the DOP challenge. However, these solvents areexpensive and usually dangerous.

None of the prior art teaches how to improve the performance ofelectrets for liquid mists other than DOP. None of the prior art teacheshow to improve the performance of an electret filtration process for aspecific liquid in gas mist challenge other than DOP. The DOP testingcriteria limits the number and type of chemicals that may be used ascharge additives and is therefore, unduly restrictive.

A method of making an electret for a target liquid contaminant in atarget gas at target conditions is needed.

Electret Improvement by Surface Treatments

The filtration properties of electret webs can be improved by applyingsurface modifying chemicals to the surfaces that interface with thefluid being filtered. Chou et al. in U.S. Patent Application Publication20030054716 teaches the swelling of an electret filter substrate polymerwith a solution containing a filtration enhancing additive. Uponevaporation of the solvent, additives are left behind within the polymerand on its surface. While this is an efficacious process, it has thedisadvantage of requiring the use of costly and often dangerous solventsalong with the costly step of drying the solvent from the substrate.Additionally, using solvents may be environmentally harmful. Evaporationand loss of the solvents by drying consumes these expensive materials.

In U.S. Pat. No. 6,802,315, it is taught to produce electret media usingvapor condensed coatings on the surfaces of the fibers. Improvedfiltration properties are achieved. Here, the range of surface coatingcompositions is limited to coating precursor monomers that may beevaporated. The process can be costly.

Other patents teach chemical modification with reactive plasma orgaseous reactants. In U.S. Pat. No. 6,660,210, surface fluorination isused to produce modified and improved electret performance. This processlimits surface modification to only fluorination and substrates that maybe fluorinated. Fluorination usually involves hazardous chemicals andexpensive equipment.

New and more flexible methods of applying surface modifying chemicalsand filtration modifying species are needed to overcome the limitationsof known methods.

Electrostatic Neutralization and Control

This subsection of art deals with the field of reducing and neutralizingelectrostatic charge on dielectric and other materials.

One may speak of surface modification in terms of the energy expendedper unit area of modified surface. On the low energy extreme, it isdesirable to neutralize static charge on substrates. Surface and volumecharge on a dielectric material can exist as a net or monopole chargeand/or as dipoles of charge in isolated regions. Accumulation of suchcharge can occur in a wide number of circumstances and with a wide rangeof dielectric material forms such as thin films, webs, sheets, fibersand threads. These may be made of paper, plastic, textiles, etc. Staticcharge is generally always present to some degree and nearly impossibleto avoid. In sheet or web transporting, it is well known that electricalcharges can build up on non-conductive materials. In industry thepresence of charges is detrimental in at least three different ways.They may create safety hazard problems. They may interfere with productor process functions, or they may contribute to surface contamination.

Regardless of the form of the material, the accumulation of net chargeon a dielectric material presents potential electrostatic hazards thatoften need to be eliminated or significantly reduced. For example,reduction or elimination of net charge is important during operation inhazardous environments, such as with a charged web moving in proximityto explosive vapors. Charge densities may spontaneously generateelectrostatic discharges and ignite the flammable vapors. Electricdischarges from substrates especially at web winding stations canproduce arcing discharges that are hazardous to operating personnel.Neutralizing charges on sheets or webs is also necessary to facilitatetrouble free passage and directing of web or sheets through processingequipment, especially in the stacking and collating processes. This isoften referred to as the elimination of static cling.

Control of substrate surface charge is important in the process ofcoating a continuously traveling web support with compositions such asphotographic emulsions, magnetic coating compositions, functionalcoatings for liquid crystal display screens, flexible electronicsubstrates, and many others. Particulate and mist contaminants areattracted to and are held on substrates by charges. It is important tominimize this in the production of photographic light-sensitiveproducts, printing plates, pressure-sensitive copying papers, lightemitting diodes, electronic substrate precursors, light display screens,optical products, etc. Clean substrates are essential in the manufactureof electronic and optical surfaces. Contaminants are a prime source ofproduct defects even for those manufactured in so-called clean rooms. Inthe manufacture of many of today's sophisticated new products withoptical or electronic functionality, the presence of even very smalldifferences in charge or uniformity of charge may create defects in thedeposition of materials or the localized functionality of the product.Such situations are not unique to those products where a plastic orpaper material is employed, but similarly apply to those products wherea glass plate, semiconductor wafer or ceramic substrate is employed. Anexample is a glass base plate for a liquid crystal display or the like.The need for improved neutralization of charge is growing ever moredemanding and important.

In general, handling webs of dielectric materials generates staticelectric charge in the material. It is well known and referred to as thetriboelectric effect. When two members are moved relative to each other,the frictional contact between the surfaces generates a static electriccharge on the surfaces. The separation of two surfaces in intimatecontact will also generate charges. For example, the simple process ofweb movement around a roller without slippage will generateelectrostatic charging. In web processing industries, static chargecauses difficulties as described above. The processes of roll formation,slitting, coating, functionalizing or laminating are troubled by staticcharge.

Troublesome electrostatic charges on charge retaining materials may begrouped into two categories. One category is that of polarizationcharges or dipoles, and the other is free surface charges. Polarizationcharges are bound to a definite site in a solid, whereas free surfacecharges are not. Free surface charges on a moving web are frequentlyreduced by a grounded brush-like device such as that described in U.S.Pat. No. 3,757,164 to Binkowski.

Polarization charges in a web are commonly controlled by subjecting theweb to a corona-generated electrostatic field having a particularmagnitude and polarity. It is often necessary to deal with bothcategories of charges. Often with polarization charges or dipoles, thereare combinations of both positive and negative charges.

Much effort over the last fifty years has been expended in providingclouds of positive and negative ions which are attracted to therespective oppositely charged areas on a substrate. U.S. Pat. No.983,536 discloses a static neutralizing device wherein an insulatedconductor with large surface area is positioned over a moving web ofdielectric material and is impressed with a high AC voltage.

U.S. Pat. No. 3,364,726 discloses a static neutralizing device whereinan insulated fine wire electrode is impressed with AC voltages atvarious frequencies ranging from 300 to 2000 Hertz, depending upon thespeed of the web material to be neutralized. The fine wire electrode isrequired to be positioned very near to the moving web, and there is alsoa requirement for a conductive metallic ground bar to be positionednearby. This type of device creates a cloud of both positively andnegatively charge species which are attracted to oppositely chargeregions on the web. AC ionizers leave a frequency signature of charge ona moving substrate that can cause non-uniformities and are incapable ofreducing substrate charge to near zero values.

Kisler in U.S. Pat. No. 4,363,070 teaches the use of a brush-like deviceof conductive filaments powered by an AC potential, to provide thecharged active species. Many wire and needle devices and improvements tothem, the methods of using them, and the methods of controlling themhave been invented. These include the teachings of Halleck in U.S. Pat.No. 4,729,057, Durkin in U.S. Pat. No. 5,432,454, Pitel et al. in U.S.Pat. No. 5,930,105, Wright et al. in U.S. Pat. No. 5,017,876, Steinmanet al. in U.S. Pat. No. 4,951,172, Blitshteyn in U.S. Pat. No.4,872,083, Halleck in U.S. Pat. No. 4,729,057, and Simons in U.S. Pat.No. 4,216,518. However, brush dischargers are only effective when thecharge density is high, and they only reduce charge levels from high tolower values. Residual charge remains on the substrate.

All known methods of neutralizing charges suffer from additionaldefects. If both positive and negative charges are present in the samecharge-retaining substrate and if positive and negative charges are tobe neutralized by having their charge levels reduced to zero, then theapplication of a DC-type electrostatic field having either a positive ora negative polarity will not reduce the charge level to zero.

Any device employing a corona producing wire will suffer from wirecontamination and produce non-uniform treatment along the wire. Multipleneedle devices become non-uniformly dirty with time and producenon-uniform treatment. Also, devices producing coronas may produce ozonegas which is a hazardous material. Conductive devices rubbing upon asubstrate may scratch and produce defective products. More importantly,while many of these devices are effective in reducing electric fieldstrengths from tens of thousands of volts per centimeter to thousands,this is simply not sufficient for today's products. Improved performanceis desired with field intensities reduced to near zero or to below 10volts per millimeter and below 1 volt per millimeter.

Improved static removal methods are needed to overcome the deficienciesof the known art.

Substrate Cleaning

Particulate contamination of substrates is responsible for huge volumesof scrap product, especially in the photographic, electronic and opticalindustries. Static charge on the substrate attracts particles and holdsthem tenaciously to the surface. Removal of static charge and webcleaning are both essential for defect reduction in manufacturing.

Takahashi et al. in U.S. Pat. No. 6,176,245 teach a series of apparatusfor cleaning and charge removal: a first apparatus for the applicationand a second for the partial removal of a cleaning organic solventmixture. The second generates static electric charge during the removalstep. This charging is diminished by the immediate application of anunder coat solution containing an organic solvent and resin compositionwhich when dried, produces a coating for some functional purpose such asa protective coating, a magnetic coating, etc. This operation usesexpensive and hazardous solvents, two separate application devices, andleaves a functional coating on the substrate.

Many methods for removing particles from the surface of a web are knownincluding air knives, suction cleaning systems, wipes, and particletransfer rollers. In non-contact web cleaners, air at high velocity ispassed over the surface of the web to remove particles. It is alsocommon to attempt neutralization of web surface charge prior to cleaningto reduce the attractive forces between particles and the web.Unfortunately, complete neutralization is difficult and not achieved.U.S. Pat. Nos. 2,980,933, 4,213,167, 5,421,901 and 4,454,621 disclosedevices for employing air streams and modification of the electrostaticcharge on the web and/or particles. The results are often notsatisfactory, and the use of tacky surface, contact cleaning systemshave attempted to produce improvements. Tacky contacting surfacesproduce detrimental static charging by contact. Non-contacting methodsare less effective than the contact methods.

A particle removal roller typically has an adhesive or tacky surface towhich particles from the web surface adhere upon contact. As theparticles accumulate on the roll, the roll becomes contaminated and mustbe cleaned periodically to restore and renew its effectiveness.Contacting the web with a roll or mechanical wipes produces staticcharging which is counter productive as this charging will attract moreparticles from the environment. U.S. Pat. No. 5,930,857 teachesimprovements to the contact roll method as do many other patents notedin its prior art description. Still, the contact cleaning apparatus mayalso scratch the substrate surface further degrading quality.

Ernst et al. in U.S. Pat. No. 5,425,813 teach the wet cleaning of acleaning contact roll while it is disengaged from performing the webcleaning function. Here, a cleaning solution of alcohol and water isused to wet fabric wipes which clean the contact roll. The contact rollrequires drying before reengagement with the web. This design requiresat least two complete systems to provide continuous web cleaning of arunning web.

Improved cleaning methods are needed to overcome the deficiencies of theknown art.

BRIEF SUMMARY

Improved Hydrocharging for Production of Electrets

One object of the present invention is to solve the problems of theprior art and provide a manufacturing method and apparatus for producingelectret articles and media by improved liquid hydrotreatments. Theinvention produces electret articles at low cost with minimum pollutionand minimum hazard and allows processing of continuous web. The priorart has difficulty contacting all the article or substrate surfaces,internal surfaces, pore surfaces and surfaces of voids with thetreatment liquid. This invention overcomes these deficiencies.

A prime embodiment of the invention is a method and apparatus comprisingusing a combination of functional liquid and functional gas andreplacing or flushing the gas from the surfaces, pores, internal volumesand voids of a substrate using the functional liquid followed by adrying step. Upon drying an electret substrate material is formed. Theuse of a functional gas maximizes the contact of substrate surfaces withfunctional liquid. It is a further teaching to enhance the mobility ofthe fluids used in the method of electret formation by using lowCapillary Number processing conditions. An advantage of this method isthat the complicated or incomplete wetting steps used in prior art areeliminated.

A prime embodiment of the invention is a method and apparatus comprisingflushing the air from the surface and voids of a substrate and airtrapped within a substrate. Flushing uses a functional gas, and it isfollowed by the replacement of the functional gas on and in the articlewith a functional liquid. This is followed by a drying step. Upon dryingan electret substrate material is formed.

Another teaching of this invention is to create electret substrate usingcontacting liquids that are deaerated and degassed so as to removedissolved and entrained noncondensible gases from the process liquid.

An additional embodiment of this invention is a simplified method ofproducing electret dielectric polymer fibers by extruding molten polymerfibers into pure steam, replacing the steam with pure water thenfollowing this with a drying step.

Still another teaching of this invention is the improved method andapparatus for making an electret comprising cleaning contaminants fromthe substrate and charging the substrate. Preferred methods andapparatus employ flushing with hot fluids.

Still another teaching of this invention is the method and apparatus formaking an electret comprising cleaning contaminants from the substrateand charging the substrate.

Still another teaching of this invention is the method and apparatus fortesting a precursor substrate or electret substrate for contamination.

Still another embodiment is the method of direct extrusion of moltenpolymer into a functional liquid followed by a drying step.

Still another embodiment is the method of direct extrusion of moltenpolymer into a functional gas followed by contacting with a functionalliquid and a drying step.

An additional embodiment of this invention is a method and apparatus formaking an electret substrate using functional gas and functional liquid,applying electrostatic charging and a drying the liquid.

Another embodiment of this invention includes the use of a chargeadditive as a constituent of a function fluid composition.

Another invention of this teaching is a method of making an electretsubstrate, media filter or article for a target liquid mist challenge,using a substrate that is not wet by the mist in the gas at theconditions of the challenge. It is a further teaching to use functionalfluids and remove functional liquid by drying. It is a still furtherteaching of this invention to modify the filtration media by a means toachieve nonwetting by the specific mist.

The article for filtering a specific mist challenge is a teaching.

Another embodiment of this invention is the method and apparatus ofhydrocharging using electrostatically charged functional liquids. Thisliquid is deposited on the substrate which is electrically isolated. Theliquid is electrically charged to an elevated potential. The liquid isdried while maintaining the wet substrate electrically isolated fromelectrical ground and maintaining a conductive path from the chargeapplying position to the drying zone through low conductivity liquid.

It is another teaching of this invention to modify the surfaces of afilter media by appending particles to the surfaces. These may beparticles of a different material, particles of an additive or particlescontaining additives, discrete particles or islands of particles orareas of a differing solid material to form a modified electretsubstrate, or electret precursor substrate.

Still another teaching of this invention is that the surface of anelectret substrate or a precursor substrate may be modified byadsorption of an additive from a fluid to form a new functionalelectret. Adsorption is the process of physical or bonding attachment ofadditive molecules, colloidal particles, aggregations, latex particles,dissolved species, colloidal phases, or dispersed phases from a fluidonto a solid surface from a dilute concentration.

In other aspects, the invention features a filter or a respirator or afiltration article that includes an electret produced according to themethods of this invention.

An additional embodiment of this invention is to contact the substratewith solid particles followed by removal of attached particles by theirphase change to a gas.

Additional embodiments of this invention are devices for implementingthe methods of this invention.

Electrostatic Neutralization and Control and Cleaning

The invention teaches a method and apparatus for charge neutralizationand its reduction on a substrate by treatment with semiconductive orconductive, grounded fluids for the cases where the liquid is wettingand nonwetting. It additionally teaches the simultaneous cleaning of thesubstrate with the conductive fluid.

Summary of Aspects

In one aspect, the invention provides a method of making an electret,comprising the steps of:

a. providing a substrate;

b. removing a first gas from the substrate with a functional, second,gas;

c. adding at least one functional liquid to the substrate; and

d. removing the functional liquid from the substrate.

In another aspect, the invention provides a method of making anelectret, comprising the steps of:

a. providing a substrate selected from the group consisting of a sheet,a piece part, an article, free particles, free fibers, and webs; and

b. hydrocharging the substrate by:

-   -   i. removing air from the substrate by immersing the substrate in        boiling water;    -   ii. removing the substrate from the boiling water; and    -   iii. removing at least some portion of the water from the        substrate by drying.

In yet another aspect, the invention provides an apparatus for making anelectret, comprising:

a. a gas applicator for applying a second, functional gas to thesubstrate to remove a first gas from the substrate;

b. a liquid applicator operatively connected to the gas applicator forapplying at least one, functional liquid to the substrate; and

c. a dryer operatively connected to the liquid applicator for removingthe functional liquid from the substrate.

In still another aspect, the invention provides a method of making anelectret, comprising the steps of:

a. providing at least one functional fluid;

b. forming a substrate directly in the functional fluid; and

c. removing a functional liquid from the substrate.

In a further aspect, the invention provides an apparatus for making anelectret, comprising:

a. a body of a predetermined volume of at least one functional fluid;

b. a forming system having an output disposed in the body of fluid forforming a substrate directly into the body of fluid;

c. a means to remove the formed substrate from the body of fluid; and

d. a means, operatively connected to receive formed substrate, forremoving the fluid from the substrate.

In still a further aspect, the invention provides a method of making anelectret comprising the steps of:

-   -   a. providing a substrate in a noncondensible gas;    -   b. replacing the noncondensible gas with a functional liquid;    -   c. selecting the functional liquid or the operating conditions        so that the noncondensible gas is functional with respect to the        functional liquid; and    -   d. removing the functional liquid.

In yet a further aspect, the invention provides a method of controllingcharge on a substrate, comprising the steps of:

a. moving a substrate;

b. applying at least one grounded liquid to at least one side of themoving substrate;

c. removing the liquid from the substrate; and

d. maintaining an electrically conductive path between a ground and theliquid until it is removed.

Another aspect of the invention is an apparatus for controlling chargeon a substrate, comprising:

a. a substrate mover for moving the substrate at a predetermined speed;

b. a liquid applicator operatively connected to the mover for applyingat least one liquid to one side of the substrate, the applicator beinggrounded; and

c. a liquid remover operatively connected to the liquid applicator forremoving the liquid from the substrate.

A further aspect of the invention is a method of modifying an electretsubstrate, comprising the steps of:

a. providing an electret substrate; and

b. supplying an additive to the substrate by contacting the substratewith a medium, whereby the electret substrate is modified by adsorbingspecies from the medium.

And, another aspect of the invention is an electret, comprising:

a. a substrate; and

b. a charge on the substrate, the charge being created by:

-   -   (i.) removing a first gas from the substrate with a functional,        second, gas;    -   (ii.) adding at least one functional liquid to the substrate;        and    -   (iii.) removing the functional liquid from the substrate.

Further objectives and advantages of my invention will become apparentfrom consideration of the drawings and the detailed description.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 illustrates an embodiment of an apparatus for manufacturing anelectret using an embodiment of the method of making an electret of theinvention.

FIG. 2 illustrates an alternative embodiment of the apparatus, includingcontactors.

FIG. 3 illustrates a further embodiment of the apparatus including asingle contactor for applying a functional fluid.

FIG. 4 illustrates yet another embodiment of the apparatus, including achamber and manifold.

FIG. 5 illustrates an alternative embodiment of a portion of thechamber, including condenser for continuously flushing air from asubstrate and continuously replacing steam with water.

FIG. 6 illustrates another embodiment including a washer element.

FIG. 7 illustrates an embodiment of an apparatus of hydrocharging usingelectrostatically charged functional liquids.

FIG. 8 illustrates a substrate charge neutralization apparatus of theinvention.

FIG. 9 illustrates another embodiment of the charge neutralizationincluding a single fluid support and dispensing element.

FIG. 10 illustrates a further embodiment of the charge neutralizationapparatus including a dip treatment station.

FIG. 11 illustrates an alternative embodiment of the apparatus forremoval of treatment liquid from the surface of the web.

FIG. 12 illustrates another embodiment of the apparatus includingopposing air doctors to effect the removal of a treatment liquid.

FIG. 13 illustrates yet another embodiment of the apparatus including adrying station.

FIG. 14 illustrates another embodiment of the apparatus for applyingwetting liquid to both sides of a film web while maintaining anelectrical path from a drying line to a ground.

FIG. 15 illustrates the surface plane of a portion of a base substratewith deposited particles and droplets.

FIG. 16 illustrates regions on a substrate.

FIG. 17 illustrates absorption modification of a substrate surface.

DETAILED DESCRIPTION Definition of Terms

An electret can be prepared according to the invention. One form of anelectret media or substrate or article consists of a solid dielectricmaterial with surfaces. On those surfaces are regions where an electricfield exists. Ideally, a great many regions exist on the surface of thematerial, and they persist for long times. The presence of an electricforce attracts and holds particulates and droplets contaminating a fluidpassing in proximity of the surface. Thus electret media are highlydesirable for filters.

To avoid confusion created by differing common and scientific usage, thefollowing terms are defined for this document:

As used herein:

-   -   “gas” refers to the gaseous state of matter;    -   “liquid” refers to the liquid state of matter;    -   “flushing” means the physical removal of material from a given        volume by displacement with another material. An example would        be the displacement of gas from a beaker by filling it with        liquid. In a similar manner air in a tube may be flushed from it        by flowing a stream of another fluid through it until no        substantial trace of the air remains.    -   “fluid” refers to material that flows and may be a liquid, a        gas, a solid particulate collection, a fluidized particle        suspension or a multiphase mixture;    -   “dihydrogen oxide” refers to the chemical compound whose        molecules are composed of two hydrogen atoms combined with one        oxygen atom;    -   “water” refers to the liquid form of dihydrogen oxide;    -   “functional liquid” refers to a charge imparting liquid which        does not wet the target substrate or device and which when        evaporated from a substrate produces an electret in the area        which was contacted by the liquid;    -   “pure water” refers to water produce by distillation,        deionization, reverse osmosis, or other purification means and        is characterized by having an electrical conductivity less than        10 micro-Siemens per centimeter;    -   “aqueous liquid” refers to a liquid mixture containing at least        10 percent water by volume;    -   “functional gas” means a gas which is highly soluble in or        absorbable into or onto a liquid, or condenses onto a liquid;    -   a gas is said to be a “gas functional with respect to a liquid”        if it is highly soluble in or absorbable into or onto the        liquid, or condenses onto the liquid and this pair of fluids is        a “functional gas-liquid pair”.    -   “polar liquid” means a liquid that has a dipole moment of at        least about 0.5 Debye and that has a dielectric constant of at        least about 10;    -   “dielectric material” means a material in which an electric        field gives rise to no net flow of electric charge but only to a        displacement of charge;    -   “nonconductive” means possessing an electrical conductivity of        less than about 100 picoSeimens per meter at the use        temperature;    -   “noncondensable gas” means a gas such, as air, nitrogen, inert        gases or oxygen that can not be condensed to a liquid without        cooling below −50 degrees centigrade at a pressure of one        atmosphere;    -   “condensable gas” refers to a gas that may be condensed to a        liquid by cooling to temperatures above −50 degrees centigrade        at a pressure of one atmosphere;    -   “steam” refers substantially pure dihydrogen oxide gas undiluted        by any noncondensable gas such as air. Steam at a temperature        above the boiling point temperature of water for the pressure at        which the steam exists is said to be “superheated”;    -   “ground” refers to an electrical ground which has substantial        ability to absorb electrical current;    -   “standard conditions” refer to as normal room conditions of 18        to 20 degrees Celsius and 1 atmosphere pressure;    -   “pure gas” means a gas of a specified molecular species        substantially free of any other species;    -   “absorb” means to suck up, engulf wholly, take in or        incorporate;    -   “volatile component” refers to species that may be evaporated        into a passing stream of gas;    -   “charge additive” means a material added to the electret target        substrate or article for the purpose of enhancing a quality of        the electret;    -   “nonwetting liquid” refers to a liquid that forms a static or        retreating contact angle of at least 45 degrees with a specified        solid surface. A perfect wetting liquid will have a zero contact        angle with a solid and tend to spontaneously spread upon that        solid surface; and for a porous substrate “nonwetting liquid”        refers to a liquid that does not absorb into the substrate;    -   “precursor substrate” is a substrate which may be processed to        become an electret substrate.        The Principle of Creation of an Electret Region by Improved        Hydrocharging

Through research and study the inventor has learned that an electretregion on a substrate may be created by hydrocharging. Hydrocharging isbest accomplished by developing a field of liquid drops of a first andsometimes a second functional liquid upon the surface of an electricallynon-conductive solid or a dielectric solid. The drops should be of amaterial that has a conductivity below 5×10⁹ picoSeimens per meter.Furthermore, the drops should not wet the surface.

Each drop is surrounded by a three phase contact line. This hinders anyelectrically charged species in the drop from moving beyond the droparea and becoming neutralized. This also prevents the charged species inand on the drop from contacting those of another drop. This hindersneutralization. The contact line prevents a charge species from exitingfrom the liquid phase without overcoming, in a thermodynamic sense, anenergy barrier. Movement of charged liquid across the contact line andacross the liquid interface is resisted by a higher energy barrier ifthe contact angle of the drop with the solid is high. Therefore, highliquid contact angle is desired to produce non-spreading drops on asurface. Also, high surface tension is desirable to resist formation ofnew wetted surface area. Note that a high surface tension does notguarantee a high contact angle, but for a given solid and gascombination, raising the surface tension of the contacting liquidgenerally moves conditions toward achieving a higher contact angle.

High surface tensions will hinder wetting of the substrate but aredesirable for electret charge generation. The preferred nonwettingfunctioning liquids are polar or aqueous, and they are pure or mixtures.Functioning liquids of this invention may contain filtration enhancingadditives.

The developing of a surface covered by a functional liquid into anelectret surface requires some amount of drying or evaporation of theliquid. The exact mechanisms of charge production on the substrate arenot known. It may be that any drops formed during the liquid removalstep have a high probability of containing an unbalanced electricalcharge. When the drop dries, the charge is concentrated into a smallerarea. Upon complete drying, the charge or its image is transferred to orleft behind on the surface or in the substrate. This concentrated chargemay be injected into trapping levels for charge carriers or inadequately deep potential wells for ions and dipole molecules.

The teaching of this invention includes methods for covering the entiredielectric solid surface with a liquid that does not want to wet it.This includes the substrate surface in any voids, pits, pores and thelike that are normally filled with trapped gas.

A wetting liquid is not desired for creation of an electret duringdrying, but it is desirable to achieve contact with all media surfacesfor treatment. This contradiction in properties for a hydrochargingliquid is one basis of prior art deficiencies.

A deficiency of prior art using fibrous web and hydrocharging liquidsarises because it is difficult to obtaining complete contact of all thefiber surface areas when using an economical functional liquid such aswater. This is especially true when trying to contact all surfaces ofthe bulk of a substrate of substantial thickness where the greatpercentage of the fibers are not immediately adjacent to an exposedface. Water commonly does not wet the fiber or substrate produced frompolyolefins and many other organics. Many electret precursor materialsintentionally contain extremely low energy surface compositions,commonly fluorochemicls, that prevent wetting. Achieving completecontacting of low energy and fluorochemical low energy modifiedsubstrates is nearly impossible to achieve with water without theteachings of this invention.

Complete contacting is further complicated with porous media. Even whenlower surface tension liquids are used or the tension of water islowered to achieve wetting of the media, these still do not completelycontact the substrate. Air is trapped in dead end pits, voids, pores,and regions. Air trapped within the media prevents total exposure of allsurfaces to the liquid prior to drying.

Impingement of the substrate with a nonwetting liquid such as purewater, or sucking liquid through or into contact with substrate requiresexpenditure of energy but still leaves many areas uncovered. Condensingnonwetting liquid from a gas phase that contains air or noncondensiblegases also does not allow total treatment of all surfaces. Dropletsnucleate directly on the surface without creating complete coverage.This invention overcomes the prior art deficiencies.

An efficacious field of drops is created by first displacing anoncondensible first gas usually air from the target's solid surfaces.This is followed by replacing the first gas, with a second gas which isa functional gas, and then replacing the functional gas with afunctional liquid where the gas functionally is with respect to theliquid. Liquid films should cover substantially all the surfaces andthen be allowed or be forced to break down to form drops. Any surfaceleft uncovered will lack the drop treatment. It is the teaching of thisinvention that the second gas be soluble in the nonwetting functionalliquid. It is an alternative teaching of this invention that the secondgas be absorbable by the nonwetting functional liquid or condense on theliquid and most preferred, the second gas be produced by boiling thenonwetting functional liquid.

A key requirement of this invention is removing gas or air from thetarget surface and covering the surface with a nonwetting functionalliquid. This is an important step in treating all surfaces of thesubstrate for conversion to an electret. Cloth fabrics, nonwovensubstrates, and fibrous mats, particulate mats and porous media are allcharacterized as having a length, a width and a thickness. Removing airfrom the inside of the thickness of a substrate and contacting all ofthe surface area of the internal media structure with functional liquidis particularly difficult.

First Embodiment and Detailed Mechanisms

In the first embodiment of the invention, an electret of this inventionis created by a multistep process. First is the replacement ofsubstantially all the air or noncondensible gases on the target surfaceregions on and within a substrate with a functioning liquid. Thisincludes the gas in voids, pits, pores and the like on, in or connectedto the target surface so that the liquid covers all surfaces within thefabric or substrate.

A second step is the removal or displacement of some portion of theliquid to create contact areas or sub regions within the target surfaceregions. These sub-regions are enclosed by single or multiple threephase contact lines, separating liquid covered and uncovered areas. Anexample of this is the spontaneous breakdown of a continuous coveringlayer of liquid into a distribution of drops surrounded by previouslycovered, but now uncovered areas on a target substrate surface. Notethat a drop on the surface of a plane has one contact line surroundingit, and a small fiber passing through a drop of a larger diameter has asubregion covered by the drop confined by two contact lines. One will bepresent where the fiber enters the drop, and one will be present whereit exits from the drop.

The third step is the complete removal of the remaining liquid by aprocess that includes a drying step. The drying step is driven tocompletion with the liquid being evaporated from the solid. Electretsurfaces will be present after drying. A high density of contactsubregions per unit surface area will create a good electret surface,especially for collecting and trapping particles from a fluid stream.Best results are obtained with low surface and bulk electricalconductivity of the liquid. This inhibits charge migration during andafter the process. Liquid with a conductivity less than 5×10⁹picoSeimens per meter is preferred. Pure water is preferred for theliquid. Distilled or deionized water are preferred forms of water.

It is also a teaching of this invention to use the step of freezing theliquid on the subregions on the surfaces. The frozen liquid may beremoved with or without reforming a liquid phase. The frozen liquid maybe removed by the process of freeze drying or sublimation.

If most single drops of liquid upon a surface contain a net charge, andthe population of drops contains a near neutral charge, then some dropsmay have a net positive and some may have a net negative charge. This ofcourse is not true if charged liquid is placed upon the substrate thatis electrically isolated from ground. The evaporation of the functionalliquid from the drops concentrates any net drop charge into a smallerand smaller area. Total removal of the water leaves behind locallycharged substrate. Here again, it is important that the liquid does notwet the solid, and a preferred contact angle is greater than 90 degreesas this will generally result in a greatest population density of dropsupon the surface. Angles greater than 90 degrees have larger volume tosolid surface contact area ratios which is believed to benefit electretproduction.

When a thin layer of nonwetting liquid coats a plane surface or acylinder (fiber), the well known fluid dynamic Rayleigh instabilityresults, and the continuous film or sheet breaks up into drops. Theprocess is driven by surface tension which produces a force to minimizethe liquid surface area. The speed of the process is controlled by theratio surface tension driving forces to viscous flow retarding forces.Here, a high ratio of liquid surface tension to liquid viscosity isimportant in improving the process of break up of a covering film intodrops. To this end, the use of water at or near its boiling point ispreferred where the viscosity is 6.33 times lower than at its freezingpoint.

The Problem Removal of Air from Porous Substrates

Covering the surface of a solid sheet with a nonwetting liquid isaccomplished by submersion. With porous substrates, simple submersiondoes not treat all internal surfaces. The process challenge is to treatthe total media surface area with a nonwetting functional liquidrequired for creation of an electret. Key to creation of a mosteffective electret substrate is to first cover completely the solidsurfaces with films of the charge producing functional liquid. Anythingless than complete coverage will limit the total extent ofelectrification of the substrate. This total coverage is most difficultwith fibrous, porous fabrics or porous media in general, because air iseasily trapped within them and is difficult to displace with liquid.

The displacement of gas is only partially accomplished by known priorart. This invention teaches substantially total removal of all air orgas from all substrate external and internal surfaces followed bycoverage with a functional liquid.

Prior art teaches the use of a wetting liquid with a low surface tensioncomposition followed by substituting with a nonwetting liquid. It willcreate three phase wetting lines that will tend to spread across thesubstrate surface. However, air will still be trapped in voids, pits andmultiple irregularities in and on the substrate leaving surfaceuntreated. This is especially true with fibrous substrates of measurablethickness. Pockets of air tend to be trapped within the thickness of themedia.

Even if the contact angle of a treatment liquid is near zero, air willstill be trapped in dead ended voids and pits in the substrate leavingsurface areas untreated. The surface area within these unwetted regionsis often quite substantial for cloth-like fabrics and porous media ofsubstantial thickness. As the surface tension of the liquid used israised, the wetting process becomes less and less efficient and thedegree of coverage of the substrate surfaces is lower and lower.

Additionally, any method of creating an electret by starting with awetting liquid requires a transition in the wetting properties of theliquid treatments, from wetting to nonwetting, after the partialcoverage of the substrate occurs. This is because the retraction ordrainage of wetting drops leaves behind thin liquid films of which allowcharge mobility. Thus, the transition of liquid properties from wettingto nonwetting is needed during processing with a wetting liquid, andthis adds complexity.

Without special methods, any emersion in a wetting or nonwetting liquidalways traps gas in the voids and pores and internal substrate regions.Complete coverage of all surfaces and filling of all voids and all poreswith liquid is best accomplished by removing the air and replacing itwith a functional gas, and then replacing the functional gas with afunctional liquid. Here again, this means the gas functionality is withrespect to the functional liquid employed.

Condensing a nonwetting liquid onto a substrate from a gas phasecontaining noncondensible air does not produce complete liquid coverageof the surface. The nonwetting liquid will deposit as drops leaving someareas, especially voids and pores untouched by liquid. Complete coverageof the target surface with functional liquid is desired.

Removal of Gas from Porous Substrates—Problem Solution

This invention teaches a method for total coverage of porous substratesand complete coverage of all their surfaces with nonwetting water orother functional liquids. It teaches methods of filling the pits, voidsand pores with functional liquid.

The process requires the first step of replacing the air from thesurface and the voids of the substrate with a functional fluid composedof molecules that are absorbed by the nonwetting liquid. The fluid isdisplaced or replaced from the substrate by the flushing action of aflow or contact with a nonwetting functional liquid. The preferred fluidis one that is absorbed into the functional liquid or reacts with thefunctional liquid in any manner where it becomes incorporated in thefunctional liquid, leaving substantially no gas phase behind. Apreferred fluid is absorbed into or onto the functional liquid.

The exact details of the process are not totally clear since part of theprocess takes place at microscopic and submicroscopic dimensions whichare not observable. In general, it is believed that a gas is preferredfor flushing air from a substrate. A gas has very high mobility, isinfinitely miscible with another gas, has very low viscosity and as aconsequence, is ideal for flushing air and replacing air from a volume.

Functional gases are identifiable as those that are much more soluble ina functional fluid than air. A soluble functional gas in a volume willdissolve in or absorb on a functional liquid and can be removed from thevolume by contacting it with the functional liquid. For example, carbondioxide gas has a solubility in units of moles per mole of liquid oftwenty nine times greater than air at 60 degrees Celsius. A functionalgas such as carbon dioxide generally is not present in the functionalliquid at a saturation level, whereas air generally saturates the liquidif special precautions are not used.

Flushing the functional gas from the substrate with a sufficient volumeof functional liquid will leave, in the beginning of the process only, asmall amount of gas behind in the pits and voids of the substrate. Theremaining gas will be bubbles in the fabric and attached to thesubstrate fiber surfaces. The gas in the bubbles will rapidly dissolvein, absorb in, or react with an unsaturated functional liquid. Again ithappens when the gas is a functional gas with respect to this liquid.The bubbles will collapse and disappear. This leaves all surfacescovered with the nonwetting functional liquid.

Surprisingly, it has been found that the functional gas flushing stepmay be eliminated if replacement and flushing of the air within thesubstrate is accomplished with a liquid in which air is rapidlyabsorbed. Such a liquid is a functional liquid from which air has beenremoved, leaving it unsaturated with air. This liquid preparation makesair a functional gas with respect to the liquid. Preferred is afunctional liquid which has a dissolved air contact as low aspracticable and at least below fifty percent of its saturation value.Many ways are available for providing liquid substantially free ofdissolved air. Functional liquid with an air content below thesaturation level for the temperature and pressure at which the liquidflushing and gas replacement takes place may be formed in many ways.More preferred is liquid with a dissolved air concentration ninetypercent below its saturation level. In all these cases a liquid has beenspecially chosen so as to make the air have gas functionality withrespect to the liquid. That is to say we have made the air and liquidhere a functional gas-liquid combination.

Most preferred is a liquid with a low dissolved air content and a highcapacity to absorb air. This is best described as a liquid having alarge difference between the equilibrium solubility and the actualamount of dissolved gas per unit volume of the liquid at the flushingconditions. Most preferred is a functional liquid which has a dissolvedair content below fifty percent of its saturation value, and an airsaturation value of at least 0.015 moles of solute per mole offunctional liquid.

In this situation, the method includes the steps of providing asubstrate containing a preferred functional gas, replacing or flushingthe gas from the substrate with a functional liquid which is notsaturated with the gas, and drying the liquid. Here again, the gasfunctionality is with respect to this liquid. Preferred is a liquidsubstantially free of entrained or dissolved gas. It is also a teachingthat the flushing with liquid may take place in two steps. The first iswith liquid which has a relatively high capacity to absorb the gas. Thisis followed by a liquid that has a lower capacity.

This invention is further illustrated by the following example. Air inan electret precursor fabric is flushed from the fabric with thefunctional liquid, pure water, with a dissolved air content below itssaturation level for the temperature and pressure of the flushing step.This water may be produced in many ways. Vacuum distillation performedat a temperature above the temperature of the flushing process willproduce the low dissolved air content water. This water is then storedand provided to the flushing without exposing it to air. Followingliquid replacement of the gas, the fabric is transformed to an electretby drying.

A functional gas may also be one that when absorbed by a firstfunctional liquid, forms a second functional liquid mixture. It may alsobe one that forms a second functional liquid upon contact with the firstfunctional liquid. A functional gas, when dissolved in a firstfunctional liquid, forms a second functional liquid mixture. Afunctional gas, when transformed to a liquid and mixed with a firstfunctional liquid, forms a second functional liquid mixture.

The removal of functional gas in the substrate leaves solid surfacescovered by the nonwetting functional liquid, even though it would not byitself spontaneously wet the substrate if place upon a portion of itssurface.

The functional gas is flushed or displaced from the substrate throughthe application or flow of the functional liquid. Any trapped volume ofgas present during the initial phase of the liquid flushing step willsubsequently be absorbed by, and become part of, the composition of thefunctional liquid. The incorporation of the functional gas into thefunctional liquid, as noted before, essentially forms a functionalliquid mixture with a slightly different composition. In the case wheresteam is the functional gas, any trapped gas bubbles will be absorbed bythe water. Water that is not boiling has the ability to totally absorbthe steam. This eliminates all trapped volumes of the functional gas.When contacted with an excess of liquid in this manner, total coverageof the surfaces of the substrate with nonwetting liquid is accomplished.

Illustrative Details of a First Embodiment

A simple application of this method of treatment is illustrated inFIG. 1. Vessel 12 is supported and cooled by heat sink element 16. Afunctional liquid 24, pure water, is present in the vessel. The liquidis pumped into the vessel through pipe 25 and exits through pipe 26.

A functional gas is used for processing. This functional gas is a gasthat when contacted by the water in the vessel, will be absorbed by it.Functional gas is present in the vessel volume gas space 10 above purewater 24. Functional gas is introduced from a source not shown by pipe23. Its continuous introduction causes it to flow out through the neck22 of vessel 12 and exit through its open top, as indicated by the arrow20. Substrate fabric 18, which is to be converted into an electret andis not wet by pure water, is placed in the flowing stream of functionalgas exiting from the vessel neck. The fabric may be, for example, madefrom material like melt blown polypropylene microfibers. After asufficient period of time, the flowing functional gas will flush all airfrom the surfaces and the volumes in and around the fabric.

After flushing with the functional gas, the fabric 18 is plunged intothe water 24 and held there. The submersion process flushes functionalgas from the fabric, and any remaining functional gas trapped within thefabric is absorbed by the water. All surfaces of the fibers of thefabric and the spaces between them will be covered with water when thisoccurs. All spaces between the fibers will be filled with water. Thisprovides coverage of the substrates fibers with the functional liquidincluding voids, pits, internal spaces, pores, and porous areas withinthe fabric. Removal of the wet fabric from the vessel and drying createsan electret article.

The method of this invention may also be preformed with the apparatus ofFIG. 1, modified to provide transport of a continuous web into the neck22, through the gas space 10 into the water 24, out of the water, andback out of the neck 22, and then on to a drying station. This is easilyaccomplished through the use of web bearing idler rolls, fluid directingand transport devices such as those described below.

The method of this invention is further understood by referring to FIG.2 which illustrates a different continuous contacting apparatus in atreatment station 100 of this invention, for cleaning and conversion ofthe substrate to an electret. At this station, a web 120 of a nonwovensubstrate is brought into contact with a dispensing device 102. Here afunctional gas is dispensed into, on and through the nonwoven to flushand displace air from the substrate. Additionally, it is the purpose ofthe gas to heat the substrate. The heated gas is produced by a means notshown. The gas is ducted to the dispenser 102 by process lines that arenot shown. Details of one form of dispenser combining the functions of102 and 104 are illustrated in FIG. 3.

From device 102 the web passes onto a water applying device 104. It ispreferred that the two contactors 102 and 104 be positioned adjacent toeach other so as to minimize the distance between them. It is preferredthat the functional liquid is also heated. During application, the hotfunctional liquid is force on and into the substrate. The functional gasis absorbable by the functional liquid. The functional liquid displacesthe functional gas and absorbs any gas not physically displaced. Excessfunctional liquid may be applied and the overflow will drip into catchpan 106.

The heated functional liquid provides cleaning of the substrate which isanother teaching of this invention. Additional heating is preferred byusing a heated functional gas.

Although liquid and gaseous dihydrogen oxide are the most preferredfluids, many other functional fluids may be used to create an electretsubstrate. These include both polar and aqueous fluids. The functionalgas may also include gases that are very soluble in the liquid beingused or gases that will be highly absorbed by the functional liquid inany manner. Functional gas may be produced by evaporation from a liquid.Many useful combinations of functional gas and functional liquid may beused. These include, but are not limited, to examples such as hot airand cold water substantially free of dissolved air, acetone gas andWater, and carbon dioxide gas and water. In every case, the functionalliquid is required to have the ability to absorb the functional gas andstill function as functional liquid, with the absorbed gas present. Ifthe liquid does not function as a functional liquid with the absorbedgas present, it is a teaching to employ a method of removal of the gasbetween the gas absorption step and the drying step.

An alternative teaching is to flush the non-functional liquid from thesubstrate with a functional liquid following the gas absorption step.There is also the case of replacing the gas in a substrate with afunctional liquid that is not saturated with the gas and will absorb thegas, thus avoiding the flushing one gas with another.

In the process of flushing air from the substrate, the volumes offunctional gas used are large enough to strip substantially all the airfrom the substrate. The volume will necessarily depend upon suchsubstrate characteristics as the bulk density, porosity, mass per unitarea, transit speed, surface characteristics, etc. Superheated steam isa preferred gas. The volume of functional liquid used will also dependupon these variables. It is often preferred that the liquid be heated.

Steam and water are inexpensive functional fluids for the process. Evenif the water is at a temperature near its boiling point beforecontacting the substrate, the steam is absorbed by the water.Surprisingly, water at its boiling point at the temperature and pressureof treatment, will absorb steam in this process, leaving all surfacescovered by nonwetting water. It is thought that bubbles of steam willnot remain in equilibrium with the water because of surface tensioneffects.

Upon passing from the functional liquid applicator 104, functionalliquid may be mechanically removed from the web 120 by devices notshown. These include gas jets to blow liquid from the totally coveredweb. Gravitational drainage, centrifugal removal, suction nozzles tosuck functional liquid from the web, squeeze rolls to squeeze functionalliquid from the web, sonic or ultrasonic vibration devices to vibratefunctional liquid from the web, or other means to remove excessfunctional liquid from the web without requiring evaporation, are ateaching. This step is not required, but, is preferred to reduce theenergy required in removing functional liquid during the drying step.

The gas in a blowing removal of functional liquid step may be athermodynamically saturate gaseous species that would be liquid at thestandard conditions, or a superheated gaseous species that would beliquid at the standard conditions. In many cases, steam or superheatedsteam will be the preferred gas for blowing gas jets. This blowing stepwill clean volatile contaminants from the substrate. Examples of whichare oil, melt processing degradation components or other contaminantresidues from the manufacture of the substrate.

The use of a heated gas in the blowing liquid removal step is preferredbecause it preheats the web prior to the removal of the remainingfunctional liquid by drying. From the functional applicator 104, the webtravels to a drier 112, where the remaining liquid is removed byevaporation. Many methods of drying are known to those skilled in theart. Surprisingly, it is found that drops of functional liquid willexist and persist on an electret substrate precursor in the presence ofsteam or superheated stream. Superheated steam may be used as the heatedgas in a drying oven. This use provides further cleaning of thesubstrate.

In transporting the web from the liquid application station to andthrough the drying step, it is preferred not to touch the web withgrounded, conductive substrate transporting structures such as idlerrolls. Rolls used to facilitate the directing and movement of the webthrough the processing station should be electrically isolated fromelectrical ground.

Two separate applicator devices are shown in FIG. 2. These may be ofvarious constructions familiar to those skilled in the art of liquid andgas application to webs. The sequential process may also be accomplishedin a single device combining both processes. The liquid applicator maybe a roll applicator, a die applicator, a flow bar or employ any of thecoating methods and devices known. Examples of coating processes aredescribed in the book “Liquid Film Coating” edited by S. F. Kistler andP. M. Schweizer, Chapman & Hall, New York, 1997, ISBN0412064812.

FIG. 3 illustrates a functional fluid applicator device 200 whichsequentially contacts a porous or nonwoven web with functional gas todisplace air from the web, contacts the web with a functional liquid todisplace the functional gas from the web, and then blows liquid from theweb prior to drying. Functional gas, functional liquid, and a blowinggas are brought into contact with web 202 traveling in a directionindicated by arrow 240 at applicator device 200.

The functional gas is flowed to chamber 216 and passes through wall 218,into the space 219 between the web 202 and contactor 230 and adjacent tochamber 216. It is preferred that contactor 230 be constructed from aporous metal material. A functional gas is supplied to space 219 at arate sufficient to allow web 202 to pass around contactor 230 withoutexcessive friction. A rate sufficient to fully support web 202 spacedoff contactor 230 and prevent touching is desired. A rate sufficient toforce and flush air from the substrate, its voids and pores is desired.This rate will depend upon factors such as the tension and the speed ofthe movement of web 202, the exact geometry and flow distribution of gasthrough wall 118 and the thickness and density of the substrate. For aporous nonwoven web, the gas will pass through and issue from thesurface of the web opposite the contactor.

Member 232 is an internal structure that divides the volume insidecontactor 230 into separate chambers 214, 215, and 216. The substratepasses chambers 216, 215 and 214 in sequence. An alternativeconstruction is possible where the wall 218 rotates while the member 232remains stationary.

A functional liquid is supplied into chamber 215 and passes through wall218 into the adjacent space 220 between the web 202 and member 230. Thefunctional liquid is flowed at a rate sufficient to flush the functionalgas from the volume of the web, its surfaces, its pores and its voids.For a porous nonwoven web, the liquid will pass through and issue fromthe surface of the web opposite the contactor.

A pressurized gas is supplied into chamber 214 and passes through wall218 into the adjacent space 222 between the web 202 and member 230. Thegas is flowed at, a rate sufficient to remove some portion of thefunctional liquid from the volume of the web, its' surfaces, pores andvoids. For a porous nonwoven web, the gas will pass through and issuefrom the surface of the web opposite the contactor and in the process,remove a portion of the liquid from the web before the drying step.

An alternative process for removing a portion of the functional liquidis to apply vacuum to chamber 214. Additional chambers may be added in acontactor similar to that shown in FIG. 3. Such apparatus may combinethe process steps of both sucking and blowing.

After contacting by device 200, the wet web continues to a drying step(not shown) to complete the conversion of the substrate into an electretsubstrate.

It has been discovered that improved performance may be obtained if theliquid used in hydrocharging has been degassed and is substantially freeof air and other noncondensible gases. Presumably, this prevents thecontamination with air during contacting steps.

Preferred fluids are pure steam, and pure water. Most preferred is waterdevoid of dissolved gases and entrained non-condensable gas bubbles. Amethod of removing gas and bubbles is described by Leonard in U.S. Pat.No. 5,505,995. Still, even more preferred is heated water devoid ofbubbles and dissolved gases. The water is flowed at a rate to providedisplacement of the functional gas from the substrate and to create asubstrate whose free volume is filled with the liquid water, and allsurfaces internal and external are covered with water even though thesubstrate is not wet by water.

Polar liquids and aqueous liquids may be used for the functional liquid.A functional gas produced from boiling a polar liquid may be used incombination with the liquid. The gas produced from boiling an aqueousliquid may be used in combination with the aqueous liquid for treatment

If gravity drainage is used to remove functional liquid before thedrying step, theoretical analysis of the liquid flow shows an importantparameter controlling the drainage separation of the water from the webis the Capillary Number. It is known and defined in fluid dynamicstexts. It is defined as the product of the liquid viscosity, times theweb speed, times the inverse of the liquid surface tension. Drainagewill be improved if the Capillary Number is minimized. This is easilydone by reducing speed or more preferably, by increasing the watertemperature. Raising the temperature reduces the water viscosity muchmore rapidly than the surface tension is diminished. Thus, heatingproduces a net reduction in Capillary Number. An elevated temperature of100 degrees Celsius results in a significant reduction in the CapillaryNumber without changing web speed. This promotes drainage of water fromthe web. It also facilitates forced removal as well.

Hydrocharging Using a Two Phase Mixture

An additional embodiment of the invention simplifies the fluidcontacting steps by using a two phase mixture of functional gas that isfunctional with respect to the functional liquid used, and thefunctional liquid. The contacting steps are performed by a singledevice. Again, the gas should be substantially free of gases that willnot be absorbed by the functional liquid. This method may also use theapparatus illustrated in FIG. 3.

The two phase mixture of functional gas and functional liquid may besubstituted for the gas flowing into chamber 216, or the liquid inchamber 215, or both. It flows onto and through a nonwoven web. Itflushes the air from web and replaces it with functional fluid. Thisallows contacting all surfaces without the interference of the air. Ifthe web is a film the mixture is flowed onto its surface.

A two phase mixture contains regions of gas and regions of liquid.Forcing these through the substrate will create at least one sequence offlushing the void volume with the functional gas, followed bydisplacement of this gas with functional liquid. The gas-liquidinterface is moved through the substrate. In fact, use of a large volumeof the two phase mixture will generally insure this takes place multipletimes. Depending on the characteristics of the substrate, it may bedesirable to flush the substrate first with a gas rich two phasemixture, followed by a liquid rich mixture. Gas rich mixture means morethan fifty percent of the volume is gas. Liquid rich mixture means morethan fifty percent of the volume is liquid.

It is preferred that the two phase mixture not be in equilibrium. Or ifit is, the equilibrium is destroyed after contacting the substrate. Inthis manner the gas may be absorbed by the liquid. It is also a teachingof this invention to follow the two phase flushing step with flushingwith a functional liquid. This may be accomplished by flowing thisfunctional liquid in chamber 214 and through the adjacent wall of member230. Using liquid cooler than the mixture helps promote completecondensation and absorption of all gas.

It should be recognized that multiple devices may be used simultaneouslyor sequentially. The two phase mixture may be flowed onto each side ofthe web simultaneously or sequentially. Following the air displacementand the flushing with the mixture, the liquid is dried from the web.Water and steam mixtures are preferred for the two phase mixture.

Alternative Embodiment of Hydrocharging

A third embodiment of this invention uses a process or method whereinair is displaced or flushed from the substrate, by a condensablefunctional gas which in turn is flushed from the substrate by afunctional liquid. When the gas condenses to a liquid, that liquid andthe functional liquid produce a functional liquid mixture. Any residualfunctional gas trapped within or on the substrate is absorbed by theliquid.

FIG. 4 illustrates another apparatus of this invention. Web 301 entersthe electret treatment station 300 with the direction of motion shown byarrow 302. Tank 310 is partially filled with functional liquid 322 to alevel indicated by the liquid gas interface 308. Idler rolls 314, 312and 316 direct the web through the treatment apparatus. Tank 310contains a functional gas distributing manifold 330 which receives gasfrom a source not shown through a process pipe 332. It is preferred thatthe gas be heated. It is discharged into the tank 310 and provides aheating means for the liquid 322. Functional gas collects in the top ofthe tank at a pressure that is positive relative to the air outside thetank, and it is discharged through condensers 304 and 306.

The purpose of the condensers is to provide a means of cooling fluidsexiting from the tank. The condensers are to provide a means ofrecovering any functional liquid evaporated in the tank. They alsoprovide a restricted entrance for the web 301 to the tank. In thecondensers the gas flows from the tank end toward the air end at itstop. This prevents any air from entering the process tank 310.

The condensers have the appropriate internal structure that anyoneordinarily skilled in the art of condenser design, may specify toprovide cooled condensing surfaces and a path way for the web 301 topass through it. The passage of the web through the functional gas inthe condenser 304 and head space of tank 310 will cause the web to beheated. The air in the void spaces of the web is flushed and purged fromthe web by the upward flow of the gas into condenser 304. Functionalliquid displaces the functional gas when the web moves below the liquidlevel 308 in the tank.

The gas entering the liquid 322 from manifold 330 creates a flow of atwo phase mixture of the functional fluids above the manifold. Thisprovides heating and a cleaning process for the web. The mixture alsoprovides the contacting and flushing actions of the functional fluids.It may substitute or augment the above mentioned flushing step. The webleaving the tank through condenser 306 will contain functional liquid ora mixture of functional fluids.

Liquid is added as needed to the tank by a means not shown. Any excessaccumulation is drained by the level overflow drain 313. This drainremoves surface liquid from the tank. It is preferred that a constantflow from the drain be maintained. This will carry away processcontaminants collecting on the liquid surface 308.

Passing from the condenser 306, the web is directed to a mechanicalliquid removal device 318. This may include one or more devices such as,but not limited, to devices for blowing, sucking, scraping, draining, orsonic treatments. From 318 the web passes to a drier 320 where theremaining functional liquid is substantially dried. Many methods ofdrying are known and are selected so as to be most appropriate for thematerial being processed.

At some position between the liquid surface 308 and the exit of thedrier 320, the continuous film of functional liquid covering thesubstrate surfaces will break down and drops will form on the surfaces.This happens even though the liquid does not have wetting propertiesrelative to the substrate at the processing conditions. Pure dihydrogenoxide is preferred for the functional gas and functional liquid.

Passing web into a chamber which contains both a gas created by boilinga functional liquid and a functional liquid followed by emersion in afunctional liquid, is also a teaching of this invention. This method isvery simple to implement. Here, the condensers are not necessary if thegas is non-hazardous and the gas may be vented to the atmosphere. In itssimplest form an apparatus the condensers 304 and 306 of FIG. 4 areremoved. Functional gas issues from the openings where the web entersand leaves the tank. It is supplied at a rate sufficient to prevent airfrom entering the tank. The gas is supplied at a rate sufficient toflush air from the web. After removing the web from the tank, thefunctional liquid is dried from the web.

Still another variation of this invention would include operating tank310 with condensers and with a functional liquid level below thehorizontal path of the web 301 through the tank. Here the web 301 wouldonly be contacted by a stream of functional gas.

Once air has been flush from the substrate by the gas, the functionalgas in and functional gas near a cold condenser surface is condensed tofunctional liquid and contacted to the web. Sufficient functional gas iscondensed to treat and cover all the web surfaces with functionalliquid. Here, as with all other functional gas contacting steps in thisinvention, functional gas should not contain noncondensible gases orair. Although there are many ways to accomplish this, a simple condenserfor purpose of continuously flushing air from the substrate andcontinuously replacing that steam with water is illustrated in FIG. 5.This condenser could replace condenser 306 of FIG. 4 or condensers 304and 306. Providing functional liquid by condensing functional gas hasthe advantage of providing distilled pure functional liquid to thecontacting process. Pure, uncontaminated contacting liquid resulting isuseful in flushing contaminants from a substrate.

Web 520 enters condenser 500 through opening 510, and exits from it byexit opening 508. Opening 510 is directly within the tank 310 of FIG. 4.Only a section of the top of tank 310 of FIG. 4 is shown in FIG. 5 andlabeled 512. In the contacting condenser 500 are mounted two coolingrolls 502 and 504. Coolant is supplied internally to these at acontrolled rate by a means not shown. These rolls cool a portion of thefunctional gas entering with the web and the functional gas in the web,and condense it to functional liquid. Enough functional liquid isproduced to cover all the surfaces and fill all the voids and pores ofthe web. Functional liquid on the cooling rolls displaces the functionalgas to leave the surfaces covered with a water film. Any inadvertentlytrapped functional gas is absorbed by the functional liquid.

Excess functional liquid flowing off the cooled surfaces is collected bypan 511 and drained away for recycling or disposal.

The web is guided by the rolls 504 and 502 and the substrate runs incontact with them. Coolant is supplied at a controlled rate andfunctional gas is provided via entrance 510 at a rate such that purefunctional gas is maintained below position indicated by line 506. Abovethe line a mixture of air and functional gas and a fog of droplets mayexist. Preferred functional gas is applied at a rate sufficient toprevent air entering exit 508. Those skilled in the art will recognizethat many variations of the condenser 500 of FIG. 5 may be used forprocessing. All are within the scope and teaching of the invention.Additionally, the tank which has a top 512 shown in FIG. 5 may beeliminated. In its place an air displacing functional gas applicator maybe substituted at the entrance 510. Such a functional gas applicatorflushes air from the web with functional gas and prevents air fromentering the condenser through opening 510. Still another simplemodification would be to supply the condenser 500 internally withfunctional gas. This supply would be at a sufficient rate and suppliedin a manner so as to prevent air from entering the entrance 510. Thissupply would also be at a sufficient rate and supplied in a manner so asto flush air from the web at or before entrance 510.

Cleaning Surface Contamination from Electret Substrates

Those skilled in the art of air filtration and electret mediaperformance recognize that oil collected from a fluid stream destroysthe long term performance of the filters. Much effort has been devotedto producing oil tolerant formulations for filter media, but there hasbeen no recognition of the importance of minor amounts of contaminationfrom the web formation and electret forming processes themselves.

Such contamination may be removed by the method and apparatus of thisinvention with and without the creation of electrets. Often fibersizing, lubricants, polymer degradation products, surface active agentsand antistatic compounds will be present on filter media and fibers.These are often used to facilitate or occur in the production of fibers,webs, cloths, yarns and films used in the production of filtersubstrates. These generally have negative effects on filter media andelectret filter media formation, life and performance. Contaminants havebeen discovered to create conditions for the mobility of charge speciesallowing neutralization of electret properties. Surface activecontamination and oil contamination of the substrate during itsformation is one notable problem. Incredibly small amounts ofcontaminants may degrade electret performance.

It is a teaching of this invention to include the step of washing,flushing in general or contaminant removal prior to processing substrateinto an electret substrate. It is also a teaching of the invention toinclude a washing step in the making of other filter substrates.

It is also a teaching of this invention to provide that the additionalstep of cleaning of the substrate during electret processes. It is ateaching of this invention to include a washing or contaminant removalstep in the methods of making an electret employing wetting ornonwetting liquids. Surprisingly, flushing with nonwetting washingliquids removes contaminants.

The washing step is further illustrated by FIG. 6. Here, melt blownmicrofiber precursor electret web 1730 is transported around a fluidfloatation and treatment devices 1702, 1704, and 1706. The first device1702 is fed internally by a means not shown with a functional gas thatis functional with respect to the washing liquid applied by device 1704.It flushes air from the surfaces of the fibers in the web. From thisdevice, the web then passes around a washing treatment liquid applicator1704. This applicator is fed internally by a means not shown.

The flushing of air, the functional gas, and the washing liquidcombination allows contacting all surfaces of the substrate includingthe surfaces of pits voids, pores and internal spaces with a washingliquid that does not wet the substrate. Washing liquid is flushed awayand replaced by a functional liquid at applicator 1706. In the process,all surfaces of the substrate are contacted by the functional liquid.Using a washing liquid miscible with the functional liquid is preferred.

A washing liquid is chosen that will not wet the web, and that willabsorb, dissolve, emulsify, suspend or by any other means, displace orremove surface contaminants from the substrate. The washing liquid willabsorb any trapped functional gas. Still further, a washing liquid ischosen that is flushable with a functional liquid.

From the functional liquid flushing step taking place on device 1706,the web passes for further processing to form an electret. Its directionof movement is shown by arrow 1720. From the flushing step at device1706, the substrate passes to a functional liquid removal device 1740,and then to a drier 1750.

Another alternative is to choose a washing liquid that has all the abovementioned properties except that it wets the web. In this case, theapplicator device 1706 flushes the wetting liquid from the web, leavingbehind nonwetting functional liquid. This and the other applicators maybe of the porous tube type and are fed internally by a means not shown.Liquids flowing from the applicator devices and through and over the webare collected in pans 1712, 1714 and 1716.

The spacing of the applicators 1702, 1704 and 1706 is sufficiently closeso as to prevent infiltration of air and reintroduction of air back intothe substrate. Of course, many other apparatus geometries and types maybe used to accomplish these washing and flushing and contacting steps.Additional processing may also be included for improving the electretmaking method.

It is also a teaching of this invention to provide a method of cleaningof the substrate during electret processes in combination with one ormultiple flushing steps. It has been discovered that contacting thesubstrate with heated fluids, and contacting the substrate withfunctional gas in the gas displacement steps described in the variousembodiments of this invention, will remove contaminants. Flushing of airfrom the substrate with heated functional gas alone will removecontaminants. Contaminants and especially oily, surface active, volatileand soluble contaminants may also be cleaned from the media during thefunctional liquid flushing step and facilitated by use of heatedliquids. The removed contaminants must not recontact and redeposit uponthe substrate.

While the exact details of the cleaning processes are not known, it isbelieved that cleaning occurs because the contaminants are volatile,soluble or dispersible in the large volume of treatment fluids,especially heated fluids. The use of hot fluid phases raises the vaporpressure of very low volatility contaminants. This combined with thecontacting, flowing and flushing of volumes of functional fluids overand through the substrate, produces a thermodynamic driving force toevaporate, desorb, wash, dissolve, displace and in general, removecontaminants from the substrate or article. Additional unknown removalmechanisms may also play a role. Heat provides energy for phase andstate transitions, for surface creation, for desorption, for statechanges, and for activations. Use of heated fluids is the preferred modeof operation.

It has been found that if a pure gas free of contaminants is passed overand through a contaminated substrate, the contaminant will move into thegas. A preferring gas is heated. Although the exact details of thiscleaning action are not known, it is known that the equilibriumconcentration will be directly related to the temperature of the phase.So large volumes of gas processed through the media will strip very lowvolatility materials from the media in an effort to produce anequilibrium concentration in the gas phase. Vapor pressures increasedramatically with higher temperatures for volatile contaminants.

When the contacting and flushing processes of this invention for makingelectret substrates are used with fluids at temperatures elevated abovethe temperature of the substrate entering the process station,contaminants are cleaned from the substrate. FIGS. 1, 2, 3, and 4illustrate such processes and process apparatus. Those skilled in theart will recognize that fabricated articles may be cleaned and tested ina similar manner as the electret substrate and precursor substrate.

Contaminant removal by the functional gas may be verified by thepresence of oily surface films or contaminant layers on the liquidcollected by condensing the gas in the apparatus illustrated in FIG. 5and FIG. 4. Common and harmful contaminants lower the liquid-airinterfacial surface tension, especially if the condensed phase has ahigh surface tension. It is preferred that the functional gas have ahigh surface tension when condensed. Steam is a preferred functional gasfor this purpose. The analytical tests for liquid purity described belowmay be used to test the condensed gas.

Contaminant removal by the functional liquid may be detectable bymeasuring the surface tension and comparing it to pure functionalliquid. Because of contamination it is not always desirable to recyclethe collected liquid without purification. Surface films of contaminantswill collect on liquid surfaces such as 308 in FIG. 4 when they are lessdense or have lower surface energy than the liquid. For this reason, itis preferred to provide any liquid process tank or reservoir withsurface skimming devices or liquid over flow drains. The constantaddition of liquid to the tank and the use of a level control overflowdrain 313 serve this purpose.

Methods where the process fluids are treated to remove contaminants andwhere the processing takes place in a manner to prevent the buildup ofcontaminants originating from the substrate are taught here. Removal ofcontamination from other sources is also taught.

The presence of oily contaminants on electret media and their potentialfor removal may be indicated by the following test. Place a sample oftriple distilled, pure functional liquid water in a clean glass retortand reflux condenser apparatus that has been thoroughly rinsed with thepure water. Known cleaning processes are employed for the apparatus.Into or onto this water, place a large sample of the media. Vigorouslyboil the liquid at atmospheric pressure for 1 hour. Cool and decant thesurface volume of the water and place it in a clean Langmiur toughapparatus and measure surface tension as a function of surface area. Thetrough is available from Nima Technology Ltd, University of WarwickScience Park, Coventry, England. Examine the condenser surfaces forcontamination.

Perform the same test by adding pure functional liquid only to theretort. Perform the pure liquid test in the cleaned apparatus before andafter the web sample test. A comparison of decanted sample surfacetensions will indicate the presence of removable liquid surface tensionmodifying contaminant agents. Contaminants generally will lower thesurface tension.

Another test for contamination is to place freshly cleaned, gold testslides in the fluids. The gold surfaces will adsorb organiccontaminants. When adsorbed, organic contaminants will change thewetting characteristics. The contact angle of water will change fromzero to a positive angle. The presence of absorbed contaminants may alsobe detected with ellipsometry.

Clean glass surfaces and gold surfaces may be prepared by immersing themin fresh piranha solution for two minutes. Piranha solution consists of3 parts of 50% hydrogen peroxide and 1 part concentrated sulfuric acid.It oxidizes organic compounds. It is also extremely corrosive to skin.Wear appropriate gloves, face shield, and apron when working with this.Rinse the solution from the surfaces thoroughly with pure water and drywith pure dry nitrogen. Note water will wet these clean surfaces.

Cleaning the substrate or article, combined with air removal ordisplacement with a functional gas, gas displacement with a functionalliquid and liquid removal by drying, creates an electret media withreduced contamination.

Extension of Methods to Sheets, Piece Parts and Fabricated Articles

Those skilled in the art will recognize that the method of thisinvention may also be carried out with sheets, parts or fabricatedarticles. Examples of such articles would be filter cartridges, facemasks or others that employ electrets to improve functionality. Suchpiece parts may be treated singularly or in groups. Articles may includefabricated filter elements which are composed of filter media andsupport and positioning structures. If these additional structures arecomposed of electret chargeable materials, they can be transformed intoelectret surfaces and enhance the overall function or utility of thearticle. In treating piece parts, they are first subjected to a flow offunctional gas to displace air, followed by displacement of thefunctional gas by functional liquid, followed by mechanical removal anddrying of the functional liquid on or in the article. The piece partsprocess may be carried out continuously by the use of continuousmovement apparatus such as carrier chains or belts transporting piecesfrom one processing position to the next. The piece parts may be treateddiscontinuously, singularly or in batches by sequentially carrying outthe steps of the process. This may be done by placing the part first inone apparatus for one step, then placing the part into additionalapparatus for additional steps. This alternatively may be done byadapting an apparatus to sequentially subject the part to the processsteps.

The emersion of the piece in a vented vat containing a two phase mixtureof pure functional gas and hot functional liquid, followed by removalfrom the mixture, mechanical removal and drying, or just drying is asimple method of this invention. The two phase mixture may be created byvigorously boiling a functional liquid so that bubbles are visiblypresent in the liquid. An alternative is to sparge functional gas from agenerator directly into the vat. It is preferred that this vat has avented but confined head space and the piece part enters the vat throughthe vent opening. The rate of flow of the functional gas is sufficientlyhigh to prevent air from entering the vent. That is the head space abovethe liquid in the vat will have a pressure higher than the air outsideof the vat.

Placing a filter constructed of materials that may be modified intoelectret materials in a vat, and continuously exposing each smallincrement of its volume to a flowing or agitated two phase mixture offunctional gas and liquid, has the following characteristics. Eachvolume will be repeatedly exposed to flushing gas followed by flushingliquid. Removing the nonwetting functional liquid from the substrate bymechanical means, followed by drying will create an electret filter.Removing the nonwetting functional liquid from the substrate by dryingalone will create an electret filter.

Processing the filter with the functional fluid and providing for theremoval of contamination from the media and removal of contaminationfrom the functional fluids is also a teaching of this invention.

The above described vat system may be adapted in many ways to treatcontinuous webs and piece parts. A rolled up long length of web may beprocessed as a unit. A stock roll of web is known in the convertingindustries as a “jumbo”. It consists generally of a ribbon of webcharacterized by a width, a web thickness and a length. The ribbon iswound around a core which is generally a cylinder with one ribbon endnext to the core, and the other end terminating at the outer wrap of thejumbo. This stock roll or jumbo generally contains a large amount of webfrom which many individual pieces may be cut. Processing the jumbo as apiece part has great economic advantage over any process requiring thejumbo to be unwound, processing each unit length of web separately, andthen rewound. Even carrying out only part of the process with the web ina wound roll form can be advantageous. An example is the flushing ofnoncondensible gas with a functional gas and liquid in the jumbo form,followed by unwinding and drying as a stretched out web.

Extension of Methods to Treatment of Free Fibers

In another embodiment of the inventive method and apparatus, an electriccharge may be imparted to one or more precursor fibers used inconstructing a web or to electret precursor elements. This is carriedout by first displacing air from the surface of the free-fiber with afunctional gas, then following this by displacing the gas with afunctional liquid. This surface treatment of free-fiber takes place asthey exit a fiber forming device. The fibers are of a non-conductivepolymeric electret precursor material. Functional liquid is depositedupon them, preferably while they are not substantially assembled into aweb. The liquid contacted fibers are collected and dried. The resultingfibers are thus electrets as is any web formed from them.

Those skilled in the art will recognize that the methods of this'invention may be applied to fibers singly or in collections prior to theformation of articles or intermediate or final substrates. The methodsmay be applied to granules, particles and other solid forms prior to theformation of articles, substrates or intermediates.

In a refined embodiment of the method of making an electret web, astream of fibers is formed by extruding the fiber forming material intoa high velocity gaseous stream of a functional gas. This process, absentthe functional gas of this invention, is commonly referred to as a meltblown fiber process. The apparatus of the type described in Van A Wente,Superfine Thermoplastic Fibers, Industrial Engineering Chemistry, vol.48, pp. 1342-1346. The gaseous stream typically transports free-fiberaway from the fiber extrusion die. The length of the fiber isindeterminate. The free-fibers become randomly entangled at, orimmediately in front of, a collector. The fibers typically become soentangled that the web is transportable as a web. The method furtherrequires the displacement of the functional gas with functional liquidand drying the liquid.

The partial displacement with functional liquid also producesfunctioning electret materials.

This process may be understood in detail by reviewing the method Kubiket al. described in U.S. Pat. No. 4,215,682. Here, the melt blownprocess and the improvement using an electrical charging apparatus areincorporated here by reference. As shown in this patent, high velocityair streams exit from the fiber die essentially coaxially with themolten polymer. This air flow is key in producing the thin fibercharacteristic of melt blown fibrous webs. In the method of thisinvention the air is replaced by a functional gas of which the mostpreferred are steam and superheated steam.

In its preferred embodiment for melt blown fibers, super heated steam isused as the processing gas in the melt blown fiber die. Additionally,pure water is deposited onto the fibers while the steam surrounds thefibers. This provides the maximum surface contact of the water to thefiber without the interference of the air. The water deposition step maybe carried out in many ways. It may be sprayed; it may be jetted on ascylindrical jets or sheet jets, or the fibers may be directed by the gasonto a stagnant or flowing pond of water or falling curtain of water, ora water flooded collection screen, or a flowing stream of water, or anip of water covered rolls or belts, or a water wet suction fibercollection device, or a water wetting suction water removal device. Thewater deposition step may employ a two phase mixture of water and steam.The water contacting step is followed by drying.

The wetting of the fiber forms a more consolidated web than air laidblown fiber.

An alternative fiber production process is the spun bond process inwhich one or more continuous polymeric free-fibers are extruded onto acollector, see, for example, U.S. Pat. No. 4,340,563. Free-fibers mightalso be produced using an electrostatic spinning process as described inU.S. Pat. Nos. 4,043,331, 4,069,026, and 4,143,196, or they may beproduced by exposing a molten polymeric material to an electrostaticfield as in U.S. Pat. No. 4,230,650. During the functional gascontacting step, functional gas surrounds the fibers during theirextrusion. After surrounding the fiber with the functional gas, it iscontacted with a functional liquid. During the step of contacting withthe functional liquid, the free-fibers may be in a liquid or moltenstate, a mixture of liquid and solid states (semi-molten), or a solidstate. Drying functional liquid from the fibers produces an electret.

A fiber may be formed by directly extruding the molten polymer in to afunctional liquid without allowing any contact with air. If this isfollowed by liquid removal and drying, electret fibers will be formed.

Hydrocharging with Electrically Charged Liquid

Another embodiment of this invention is the method and apparatus ofhydrocharging using electrostatically charged functional liquids. Thisliquid is deposited on the substrate which is electrically isolated. Theliquid is electrically charged to an elevated potential. The liquid isdried while maintaining the wet substrate electrically isolated fromelectrical ground and maintaining a conductive path from the chargeapplying device to the drying zone though low conductivity liquid.

Details of this invention are illustrated in FIG. 7. The treatmentstation 600 processes web 620 whose direction of motion is indicated byarrow 662. The web first passes around cleaning apparatus, porous tube672. It may have at least some of the internal features of thatillustrated by member 230 in FIG. 3. Illustrated in FIG. 7 is chamber675. Into this chamber, heated gas is introduced to clean the web.Cleaning apparatus chamber 675 is supplied by gas, preferably afunctional gas, by a means not shown. The gas may be replaced by a twophase mixture of gas and liquid or a cleaning liquid. The gas exitsthrough the exterior surface and flows through and over the substrate620. This flow strips contaminants from the web. The flow of the gasflushes air from the substrate. Cleaning tube 672 is constructed in sucha manner so that the web and any liquid on the web are electricallyisolated from electrical ground.

The web then passes to fluid contacting and charging apparatus 667 whichhas features similar to device 230 in FIG. 3. Or, some other apparatusmay be used for performing the function of flushing air from thesubstrate with a functional gas. This is followed by replacement of thegas with liquid. The functional gas is functional with respect to theliquid. A preferred liquid is a functional liquid. Examples of preferredfluids are gaseous and liquid dihydrogen oxide. As shown, gas isintroduced into chamber 665 and liquid into chamber 664. The fluidcontacting and charging apparatus 667 is constructed in such a manner sothat the web and any liquid on the web are electrically isolated fromground. Apparatus 667 is also connected by wire 674 to an electrostaticpower supply 676. It enables electrical charging of the liquid appliedto the web with an elevated electrical voltage. The power supply 676 maybe an AC or DC voltage source. Charging may involve alternating theapplied charging between on or off, positive or negative charging, orcombinations of these. Electrical charge may also be varied in time.

Many liquids have some finite but very small electrical conductivity, orare semiconductors, and the cross sectional area of the mass of theliquid applied to the web is large. The conductive or semiconductivefluid path is maintained between the charge application position and thedrying zone. Because the electrically conductive structures of 667 arestructurally positioned in contact to the liquid on the web, the liquidcovered web will leave apparatus 667 electrically charged by the appliedelectric potential.

The functional gas-liquid combination flushing process allows coveringand treating all substrate surfaces with the liquid. The covering withfunctional liquid includes pits, voids, pores and all surfaces ofinternal spaces of the substrate.

From the charging apparatus 667, the web passes over idler rolls 666,678 and 680. These rolls direct the web to a mechanical liquid removaldevice 668 and then onto a drying device 670. The mechanical removaldevice is not required. All apparatus contacting the web must beelectrically isolated so as not to bleed off the electrostatic chargeapplied at substation 667. The electrical isolation requires that allgrounded surfaces must not touch the liquid wet web. Additionally,electrically isolation requires that all grounded surfaces must also beseparated by electrically insulating material, or sufficiently large airgaps with sufficiently high break down voltages. This preventselectrical spark discharges. Such discharges will dissipate theelectrostatic charge applied to the web.

Electrical isolation of high voltages is often difficult in a wet andhumid environment. For this reason, redundancy is preferred forelectrical isolation means and mountings.

Idler roll 680 is electrical isolate from ground. Its potential may beallowed to float and seek its own voltage level. It is preferred that680 be maintained at an electrical potential by connecting it to anelectrostatic power supply. Drying removal of the liquid will producecharged electret substrate. If the liquid dried from the web in drier670 is a nonwetting functional liquid, the process of deposition of theliquid onto the web and drying will produce charged web by itselfwithout using electrostatic charging.

At some point in the drying process, the nonwetting liquid will cease toform a continuous coating on the surfaces of the substrate. Charge isthen isolated within the drops and no continuous liquid film will bepresent to allow mobility of charge species. Mechanical removal of theliquid may be employed to assist the drying process.

This process differs in several important ways from airlesselectrostatic spraying of charged drops of a nonwetting liquid onto theweb in air. The drops will initially deposit upon the uncharged web. Asthey accumulate on the web, charge will accumulate. This chargeaccumulation will be self limiting. The charge on the web will beidentical in sign to the atomized drops approaching it. These likecharges will repel additional drops directed at the web. In this case,then only part of the surface of the web will be covered with nonwettingliquid. The presence of air also prevents complete surface coverage withthe liquid. The spraying does not force air from the surfaces and voidspaces on and in the web, and does not produce complete displacement ofair and its replacement with nonwetting liquid.

In the process of FIG. 7, many liquids may be used. Wetting liquids maybe used. Pure liquids and functional liquids are preferred. Whenfunctional liquids are used hydrocharging also may occur. It willproduce electret web even if the active electrostatic supply systempowered by voltage source 676 should fail. It will produce electret webeven if some apparatus or operational failure electrically shorts outthe charged liquid on the web. In this manner, functional electretsubstrate is produced even when the active charge application from theelectrostatic power supply fails. It is preferred that the functionalliquid be an aqueous liquid. It is most preferred the liquid be purewater.

Unique electret substrates may be made by contacting a precursorsubstrate with a functional liquid and drying it to produce an electretwhen the electrostatic charging process is employed. Alternating theapplied charge from on to off, from positive to negative, and anycombination of these, can create alternating charge characteristicsacross the electret. As noted above, when the charge is off, thehydrocharging process produces an electret.

Processing with a wetting liquid and flushing with a functional gas andfunctional liquid combination allows all surfaces including those inpits, voids, pores and internal volumes of the substrate to be treatedwith the liquid. Note that even though a liquid wets the substrate, airis commonly trapped on and within the substrate. The flushing process ofthis invention removes it.

Electrostatic charging of the liquid on the substrate may also beemployed at any point prior to drying. The charging may be achieved bydirect contact with an electrode, or electrodes, or by deposition ofcharged species directly on the liquid including charged drops,electrons or ions.

Electret Substrates for Filtering Liquid Mists

Electret substrates have within them electrically charged points, areas,or locations. This charge is important for efficient filtration in thatit attracts and holds particulates and aerosols contaminating gasstreams. The targeting of electret surface properties to variousfiltration challenges is another teaching of this invention. Also taughtis how to use of filtration additives to modify substrate properties totarget specific mist or liquid aerosol challenges, and how to choosefiltration media to meet a specified mist filtering challenge.

Previous inventors have found through experimentation and trial anderror, that using additives in the polymer of the substrate improvesmist resistance for dioctyl phthalate (DOP) mists in air at roomtemperature. Unfortunately, all prior art teaches improving an electretfilter's resistance to oily aerosols in terms of filtrationcharacteristics defined by the performance with dioctylphthalate (DOP)mists only, and only in air streams at room temperature and pressure.For example, in U.S. Pat. No. 6,802,315, samples were evaluated using aDOP loading test. The DOP test challenge was on NIOSH standards ascodified in 42 C.F.R. sctn. 84 for filter classes where the three levelsof filter efficiency are 95%, 99% and 99.97%.

All prior art teaches only how to improve filtration of DOP mists basedupon experimental observations of the effects of various constituents inthe substrate. While this is certainly useful when filtering DOP mistsin air at room temperature, and it might be useful for the problem offiltering mists of identical properties in air, it has little bearing onmodifying substrates for other filtration challenges or for pickingoptimum filter media for use in other circumstances.

The DOP filtration challenge test does not have utility for choosingworkable or economically optimized filters for other challenges.Examples are inorganic salt water mists, liquid metal mists, siliconeoil mists, fluorocarbon oil mists, synthetic lubricant mists,contaminated motor oil mists, and an almost infinite number of otherpossible mists generated by nature and industry.

The oily mist and liquid aerosol descriptive terms of the prior art donot define and quantify the true nature of the multitude of filteringchallenges that these general terms really include. For example, theterms oily mist or liquid aerosol describe a huge range of liquidmaterials that may form drops suspended in a gas. Surface tension is butone important property characterizing the wider variability of the mistfiltering challenge. This variability is partially identified by theNational Advisory Committee for Aeronautics, Technical Note 2030,“Variation with temperature of surface Tension of Lubrication Oils” bySydney Ross, February 1950, page 7. It lists “oil” surface tensionsranging from 60.9 to 29.5 dynes per centimeter at room temperature andpressure in air. The surface tension of DOP is 31.1 dynes percentimeter. An electret filter that meets the DOP-air challenge is overdesigned for the “oil mist” with a surface tension of 60.9 dynes percentimeter.

Furthermore, surface tensions for potential mists vary over a wide rangefrom 2 dynes per centimeter for liquid hydrogen to 1880 dynes percentimeter for molten iron. Surface tensions for common industrialliquids in air at 20 degrees C. in units of dynes per centimeter areperfluoroheptane 11.0, perfluoromethyl cyclohexane 15.7, heptane 19.7,methanol 22.6, toluene 28.4, propylene carbonate 41.1, dimethylsulfoxide43.5, pure water 72.2, and mercury 476. Various oils have widely varyingsurface tensions in air: new lubricating oil 35, used lubricating oil20, Castor oil 35.6, and silicone oil (Power Chemicalpolydimethylysiloxane PCC silicone oil) 15.9 to 21.5.

Another problem with the DOP-air test performed at one atmospherepressure is that it is not relevant when the gas is not air. It is notrelevant when the gas is uranium hexafluoride, nitrous oxide or Freon®.It is not relevant when the gas pressure is 0.01 atmospheres or 100atmospheres. Note that the water-carbon dioxide gas interfaceexperiences a change in surface tension of 0.7 dynes per centimeter forevery 1 atmosphere pressure increase at room temperature. (See Adamson,Physical Chemistry of Surfaces, Second Edition, Interscience Publishers,New York, 1967, page 61)

The DOP-air test at room temperature is not relevant when thetemperature is drastically raised or lowered from room temperature.Surface tension commonly decreases by 0.1 to 0.2 dynes per centimeterfor each degree Celsius rise in temperature, and the surface tensionapproaches zero near the critical temperature. See Adamson, PhysicalChemistry of Surfaces, Second Edition, Interscience Publishers, NewYork, 1967, page 57.

Mists and aerosols of many differing and complex chemical compositionspresent challenges to gas filtration media. These range from cookingoils, incomplete combustion products, commercial and home productaerosols and a never ending list of industrially generated mists ofchemicals, ranging from liquefied gases to liquid metals. Mists andaerosols of many differing and complex chemical mixtures presentfiltration media challenges in a wide variety of different industrialgas streams. The gases range from organic gases (methane,dichloroethane, Freons®), elemental gases (fluorine, helium, oxygen),inorganic gases (nitrogen dioxide, ammonia, hydrogen disulfide),vaporized inorganics (steam, silicon hydride, hydrogen disulfide), andvaporized compounds (ethanol, carbon disulfide, gasoline, chloroform).

The problems of filtering any mist from any gas have been studied.Careful research and study of liquid drop contamination of filtrationsubstrates has identified a mechanism of charge neutralization by liquidcontaminants. Past inventors have not determined the mechanisms creatingpoor or improved performance. They have only tested filters against theDOP-air challenge.

I have found that efficient electret filtration may be achieved if thefilter substrate surface is not covered by liquid. Liquid allows chargemobility which allows canceling or modifying of the effective charges ofelectret media. The mobility occurs even if the liquid is characterizedas a non-conductive. I have found that efficient electret filtration ofmists may be achieved if a filter substrate in a gas is chosen, so thatit is not wet by the mist contaminant in the gas. Wetting of a solid ina gas by a liquid is controlled by the surface energies of thesolid-gas, liquid-gas and liquid-solid interfaces. These vary greaterwith household and industrial materials.

Improved economics may be achieved if the filtration substrate isselected for a specific mist challenge at the conditions of thefiltering process. This includes the temperature, pressure and thematerial of the mist.

Contaminating drops or mists of water, or aqueous liquids will eitherwet or spread upon the substrate or collect as discrete drops upon thesubstrate. When they spread, electret charge is neutralized in theliquid contact areas. When they remain as drops, electret chargeneutralization is very limited. Another key factor that correlates withwetting is the contact angle the liquid makes at the three phase contactline on the substrate. As with wetting the contact angle is alsoeffected by the surface energy of the solid-gas surface, the surfacetension of the liquid-gas surface, and the surface energy of the liquidcovered solid surface. A high contact angle is preferred to prevent poorelectret performance.

For any water mist filter challenge, the results of a DOP challenge arenot meaningful. Any media that fails the DOP test may or may not fail awater mist test.

Electret performance may be improved by adjusting a combination ofsurface and gas properties to prevent wetting of the substrate.Generally, this corresponds to adjusting the interface properties toachieve large contact angles. Contact angles greater than forty fivedegrees are preferred. Contact angles greater than ninety degrees aremost preferred. The angle may be increased by lowering the surfaceenergy of the substrate in the gas or increasing of the surface tensionof the contaminant mist in the gas.

To understand the detrimental wetting of filter media, it should benoted that wetting is a process that is primarily influenced by thesurface energies and thermodynamics of all the phase interfaces present.These may be changed in many subtle and sometimes unexpected ways. Manyvariables can influence wetting and an exhaustive list is impossible toprepare. Luckily, wetting can be defined by practical tests that areeasily preformed to determine if the filter media is preferred for aspecific mist challenge.

Preferred wetting tests are described below. All tests are performed atthe temperature and pressure at which the filtration process will becarried out. All tests are performed with the actual gas being filtered.

For porous filtration media, a small drop of the mist forming liquid isplaced on the media. If it is absorbed into the media this is evidenceof wetting. The time scale for absorption will vary depending upon theliquid viscosity since the absorption process requires flow of theliquid. High viscosity liquids have higher resistance to flow and flowmore slowly than low viscosity liquids under the same circumstances.Liquids like water, if they wet the media, may be absorbed in the matterof seconds to minutes. Liquids of high viscosity like honey, if they wetthe media, may take minutes to hours to be absorbed.

On a solid, nonporous surface wetting may be defined by placing a smalldrop on the surface and observing its behavior with time. If the area ofcontact one minute after the depositing step is larger than the area ofcontact later, then the liquid has spread over and has wet additionalsurface areas of the substrate. For a given surface material, givenphysical surface characteristics and thermodynamic state of thematerial, the spreading is at least dependent upon both the chemicalcomposition and thermodynamic state of the gas phase, solid phase andliquid phase present. Modifying any of these states or conditions orcompositions can change whether the liquid spreads on or wets thesubstrate or does not wet. Changing, among other things, the gasmixture, the gaseous chemical make-up, contaminants on the solid, thegas temperature or the gas pressure can modify the process conditions tothe extent that spreading occurs or does not occur.

Those skilled in the art will recognize that many variations ofspreading or wetting tests may be used. They may include, but are notlimited to measurements on scales from microscopic to macroscopic, nakedeye observation and instrument assisted observation, tests of bulkmedia, or tests of specific surfaces of the media including individualfibers and elements of the media.

Modification of Electret Media to Avoid Wetting by Mist Liquids

It is a teaching of this invention to modify the filtration media by ameans to achieve nonwetting. That is the modified filter media is notwet by the liquid of a target mist-gas challenge. Modification may bemade in many ways.

One method of making a substrate for filtering a gas containing a liquidmist at a given set of conditions comprises the steps of using asubstrate; and, selecting the surface properties of the substrate suchthat the substrate is not wet by the liquid at the conditions (includingtemperature and pressure).

It is a preferred teaching of this invention that filtration additivesbe added to the gas or liquid streams contacting the electret precursorsubstrate or the solid material of the filter substrate during theelectret making process. The purpose of this addition is to modify thewetting characteristics of the electret substrate filter so that it isnot wet by the liquid of a target mist-gas challenge. These additivesmay be dispersed, dissolved or entrained in the fluid as solids,liquids, gases or supercritical fluids. These are present in smallamounts preferably below 1 percent by weight in the fluids used fortreatment. It is preferred that the additives be solid or non-mobilebefore the charge implanting step. If hydrocharging is used it ispreferred that the additives be solid or non-mobile before finalevaporative drying step. If the additive is liquid when applied to thesubstrate additional process steps which produce solidification areused. These steps may include cooling, curing, absorption, radiationprocessing, etc. The process of the solidification step will depend uponthe individual composition and characteristics of the additive.

It is another teaching of this invention that the surface of the filtermedia be chemically changed to modify the wetting characteristics of theelectret substrate filter so that it is not wet by the liquid of atarget mist-gas challenge. Chemical modification may take place bychemical reaction with a solid, liquid, gas or supercritical fluid. Thechemical reaction may be activated or forced to proceed by applicationof electromagnetic radiation, heat, pressure, plasma processes or coronaprocesses or any other means of applying any form of energy. An examplewould be the plasma fluorination of the substrate surface. Anotherexample would be the reaction and attachment of polyethylene-likematerials from reactive gases.

It is another teaching of this invention that the surface of a filtermedia may be modified by combing the major component material of thefilter media with another solid material to form a new material. Thisnew material is a macroscopic or microscopic blend of solid materialsthat it is not wet by the liquid of a target mist-gas challenge. It iswithin the scope of this teaching that this combining may include two ormore physical solid forms of the same chemical material. Examples wouldinclude blown polypropylene microfibers combined with Telfon® powder,polyolefin fiber with Teflon® fibers, and polyester fibers withpolyester powders.

The polyester surfaces may be modified by functionalizedperfluoroethers. The surface of one or more components of aheterogeneous blend may be modified by an additive such as a polyethermodified fluoroalkylsiloxane described in U.S. Pat. No. 5,908,598. It isanother teaching of this invention that the surface of a filter mediamay be modified by coating it with another substance to form a surfacethat it is not wet by the liquid of a target mist-gas challenge. Coatingof liquids or solids may be used. It is preferred that the liquids besolidified by some means. It is within the scope of this invention thatliquids may be volatile and contain nonvolatile compounds. An example ofsuch a liquid would be a solvent in which a silicone polymer isdissolved or dispersed.

Electret Modified with Particles

It is another teaching of this invention to modify the surfaces of afilter media by attaching to or placing on the surface particles,particles of a different material, particles of an additive or particlescontaining additives as discrete particles or islands of particles orareas of a differing solid material to form a composite electretsubstrate, or electret precursor substrate.

These particles may be used to modify the surface energy of the originalfilter media. It is useful to make the surface energies of the compositesurfaces non-uniform. It is useful to create a composite which is notwet. It is useful to create a composite which has a modified surfacearea per unit volume. It is useful to create a composite with surfaceproperties that are heterogeneous where these properties include, butare not limited to, chemical, thermodynamic, electrical, magnetic,radiationally interactive and physical. It is useful to create acomposite which has modified filter properties.

FIG. 15 illustrates the utility of the invention. The surface plane of aportion of a base substrate is illustrated by line 1550. On this aredeposited particles like particle 1555. The surface region identified bydashed line 1570 is heterogeneous. Liquid droplets like droplet 1560 inthe gas above the surface 1550 may be attracted to the surface 1570.When the particle 1555 and its companions are not wet by the liquid ofthe mist, the composite heterogeneous surface assumes the property ofnonwetting.

Mist droplets such as 1565 when deposited upon the composite surface,only contact the particles which they do not wet. They remain dropletsand do not spread upon the composite surface. Because of this, thecomposite surface is not wet by the mist even if the liquid of the mistwets and spreads upon the material of the base substrate 1550.

Particles are useful for modifying the filtration and wettingcharacteristics of the electret substrate. These particles may beprepared from polymer that contains electret charge stabilizingadditives or performance improvement additives. This non-uniformity on asmall scale can produce a media which is not wet by the liquid of atarget mist-gas challenge.

It is a further teaching to attach or place solid particles on thesubstrate surfaces to obstruct the advancement of liquids, and toobstruct the movement of three phase contact lines. Additionally, it isa teaching to modify the substrate surface physical or chemicalstructure to obstruct, prevent or retard the movement of liquid andwetting of the substrate by liquid. Making the surface propertiesheterogeneous is an effective means of accomplishing this. This isillustrated in FIG. 16.

FIG. 16 is a plane view looking down upon a substrate surface region1660. On the base substrate are multiple oval regions like thatidentified as 1680 which have lower surface energy than the basesubstrate. These have an energy such that they are not wet by the mistliquid. Additionally, the 1680 like regions enclosed areas of basesubstrate 1675, 1670, and 1665. The base is wet by the liquid of themist, and this liquid will spread upon its surface. Liquid from the mistis trapped in the areas 1675, 1670, and 1665, and is prevented fromfurther migration by the heterogeneous nature of the composite surface.The nonwetting properties of the 1680 regions trap and curtail unwantedmigration of liquid.

Generally, melt process fibrous polymeric electret substrates tend to besmooth surfaced fibers. The creation of substrate surface structure byappending particles may produce or improve nonwetting properties. Fornonwetting purposes these locally change the surface energy, surfacearea per unit mass, surface chemistry and surface roughness. It isbelieved that all of these pin three phase contact lines and retardtheir movement and thus, produce useful nonwetting or antimigrationproperties.

For electret enhancing additive modification of a substrate, theaddition of particles increases surface area and structure. A string ofspherical particles of a given diameter has more surface area than afiber of the same diameter and mass. Irregular shaped particlesgenerally have greater surface area than spherical ones per unit mass.Increased active surface area helps produce improved filtration.

Examples of enhancing additives are described in U.S. Pat. No.5,645,627. Other examples are the perfluoroalkylacrylate resins of U.S.Pat. No. 5,935,303 and the dibloc wax from Mammut Sports Group, AG ofSeon, Switzerland. The particles may contain additive contents rangingfrom 0.1% to 100%. The particles may be fibrous, acicular, spherical orany other shape.

An advantage of diluting media wetting characteristic modifyingadditives with polymer, and modifying the substrate with such particlesis economic. Additives are often very expensive chemicals.

The polymers used to form the fibers of the web can be selected fromamong many suitable polymers. Examples of these polymers includepolypropylene, polyethylene, polyester, polyamide, polyvinyl chloride,and polymethyl methylacrylate. The fiber diameter of the polymer used toform the polymer fiber web generally is in the range of 1 to 20micrometers. Depending on the intended application, a more preferredpolymer fiber diameter is in the range of 3 to 12 micrometers. Modifyingparticles may range in size from submicron to tens of microns.

One useful charge stabilizing additive is a fatty acid amide. Examplesof preferred fatty acid amides include stearamide and ethylenebis-stearamide. An exemplary stearamide is commercially available asUNIWAX 1750, available from UniChema Chemicals, Inc. of Chicago, Ill. Anexemplary ethylene bis-stearamide is commercially available as ACRAWAX™which is commercially available from Lonza, Inc. of Fair Lawn, N.J.

Another suitable charge stabilizing additive is a nonionic, oleophobicfluorochemical surfactant. One example of such a compound is afluorochemical urethane derivative such as a fluorochemicaloxazolidinone. Such compounds are described in U.S. Pat. No. 5,025,052,which is hereby incorporated by reference. An example of a suitablefluorochemical oxazolidinone is commercially available as SCOTCHBAN™Protector FX-1801 from Minnesota Mining and Manufacturing Company of St.Paul, Minn.

The stearic esters of perfluoroalcohols can also serve as suitablenonionic, oleophobic fluorochemical surfactants that are useful ascharge stabilizing additives. Such compounds, which can befluorochemical intermediates, can have the general structureR_(f)CH₂CH₂OOCC₁₇H.₂₆ where R_(f) is CF₃ [CF₂]_(n) where n is from 3 to17. An example of such a compound is ZONYL™, commercially available fromE.I. du Pont de Nemours & Co., of Wilmington, Del.

Particles are supplied by contacting the substrate with an appropriatemedium. Examples are dilute latex formulations, dilute emulsions, dilutesuspensions, fluids containing colloidal particles, solids suspended influidized beds, fluids containing dusts, solids suspended in gas streamsand solids suspended in supercritical fluids.

The process steps of filtering targeted additive particles with anelectret media to form a particle enhanced electret media is anotherteaching of this invention. It is preferred to contact the particlemodified media with a liquid and remove liquid with a drying step. It ispreferred to recharge the solid particle modified media to form a finalelectret media. It is most preferred to produce the final media byflushing air from the particle modified media with a functional gas,flush the functional gas with a functional liquid, and then dry thefunctional liquid from the media.

Substrate Modification by Adsorption of Additives from a Fluid

It has been discovered that the surface of an electret substrate or aprecursor substrate may be modified by adsorption of an additive from afluid to form a new functional substrate. Adsorption is the process ofphysical or bonding attachment of additive molecules, colloidalparticles, aggregations, latex particles, dissolved species, colloidalor dispersed phases from a liquid or fluid onto a solid surface from adilute concentration. In the process, the additive species migrates fromthe bulk fluid and deposits upon the media surface. In the case ofparticles, surface structure then becomes heterogeneous when observed ata microscopic level where the discrete particle can be observed. In thecase of molecules, the modified surface may be homogeneous orheterogeneous.

Various problems with other means of modifying a porous electretsubstrate of measurable thickness exist. Additives introduced to polymerprior to forming the sheets or fibers must be added to the entire massof substrate not just the areas forming surfaces. This requires muchgreater quantities of expensive modifying chemicals adding to the cost.Additionally, these must be stable during melt processing.

Coating functional additives from solutions on the surfaces of the thickporous electret substrates also has problems. Drying occurs from theouter dimensions and concentrates the dissolved additives there. Asdrying proceeds more and more solution is brought to these outer layers.When the solvent evaporates, it concentrates the dissolved additivesthere.

Reactive chemical surface treatments are difficult to achieve uniformlythroughout the thickness of a porous fabric. If the reaction isactivated by radiation, more extensive treatment on the outermostexposed surfaces commonly results. Treatment of the inner surfaces isdeficient. The radiation is shielded and often unable to penetrate theinner spaces. Without the teachings of this invention, the preferredmodification of all surfaces of the substrate surface, including thesurfaces of pits, pores, voids, and internal volumes, is problematical.

In the adsorption from dilute active species in a fluid medium,migration occurs from the bulk fluid to the surface because the additiveprefers to adsorb at the fluid-solid interface. Active species move tothe solid surfaces from the bulk fluid by a diffusional process, and theflushing of fluid through the substrate insures equal treatment of allsurfaces internal and external. When adsorption from a liquid is used,the teaching of this invention may be employed to insure all surfaces ofpits, voids, pores, and internal volumes are contacted by the additivecontaining liquid, rather than being prevented by pockets of trappedgas. In these cases, the contacting occurs even through the processliquid may not wet the substrate.

Preferred additives are those which may be prepared in diluteconcentrations in the fluids of these inventive processes. Dilutionallows the treatment fluid to penetrate the total thickness of thesubstrate for treatment of the total mass of the substrate. With surfaceadsorption, the rate of the process and degree of treatment is generallycontrolled by the bulk concentration of the adsorbing species in thefluid, and the adsorption driving forces.

Rinsing steps may be employed after the adsorption step to recoverexcess additive from the surfaces. Rinsing steps may be employed toreplace a treatment liquid composition with a liquid without additiveprior to drying. The best sequence and selection of processing stepsprior to the final drying step will depend upon the use of and thechoice of additives. Depending upon the details of the modificationmaterials, two drying steps may be required: a first drying to fix theadditive to the surfaces of the substrate and a second to producehydrocharging. Often, only one is needed.

FIG. 17 illustrates a process using absorptive modification of thesubstrate surfaces. Porous web 1750 is transported through a webcleaning processor 1751. From there it is brought to a porous tubefunctional gas contactor. The gas is supplied to internal baffled region1754 and exits through the porous wall of the contactor. Upon exiting,it flows through the substrate displacing and flushing air from thevolume of the substrate. From the gas contactor, the web passes to afunctional liquid and gas contactor 1760. A two phase mixture offunctional liquid and functional gas is flowed into internal region1756. This mixture then is directed into contact with and through thesubstrate. Functional liquid is introduced into internal region 1758 ofthe contactor 1760 and from there it is directed so as to complete thereplacement of gas from the substrate. The web issues from the liquidcontacting device saturated with liquid and devoid of substantially allgas pockets. It directly enters absorption treatment tank 1764.

Tank 1764 is filled with liquid containing a dilute concentration of afiltration enhancing additive chosen so that it will adsorb onto thesurfaces of the substrate. Preferred is a liquid that is miscible withthe liquid applied by device 1760. Most preferred is a liquid identicalto that used in 1760. From the absorption tank 1764 the web passes to arinse tank 1766 of functional liquid.

From the rinse tank 1766 the web passes to a mechanical liquid removaldevice 1768 and onto a drier 1770 to complete the process of cleaning,surface treatment, and conversion of the substrate to an electretmaterial.

Many suitable filter surface additives are known. The additives may becapable of altering the characteristics of the electret by improving theability of a polymer substrate treated therewith to maintain a chargeafter being hydrocharged (i.e., increasing the charge stability of atreated electret relative to an untreated electret), increasing theamount of charge exhibited by a treated substrate after hydrochargingrelative to the untreated substrate, change the wetting characteristicsof the treated surface, change the performance against mist challengesand combinations thereof to make a enhanced electret filter.

Suitable additives include fluorochemical additives including, e.g.,fluorinated oxazolidinones, fluorinated piperazines, perfluorinatedalkanes and fluorochemical additives disclosed in U.S. Pat. No.5,411,576. One useful commercially available charge additive ispoly[[6-(1,1,3,3,-tetramethylbutyl)amino]-s-triazine-2,4-diyl][[(2,2,6,6-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl-4-piper-idyl)imino]], which is available under thetrade designation CHIMASSORB 944 from Ciba Spezialitatenchemi AG (Basil,Switzerland). Still other additives include fluoroalkylsiloxanes,fluorinated waxes, alkyl waxes, n-docosane, n-octacosane,n-hexatriacotane, silicone polymers, silicone containing chemicals andUV polymerizable fluids such as an acrylic functional perfluoropolyetheras described in U.S. Pat. No. 4,472,480 as compound II.

Adsorbed additives may be liquids. These liquids are converted to solidsby any of the known or previously described methods.

It has been found that the addition of adsorbed particles and additivesthat have very low surface energies to a filtration substrate willimprove the filters constructed from these substrates. Examples includeparticles which have compositions that include some fluorinated carbonsatoms in their molecular structure or materials known under theregistered trade marks of Teflon™, Viton™, KelF™, and Florel™. Thesematerials are generally prepared by emulsion polymerization. Theintermediate or final emulsions may be used for electret modification.

Articles

Nonwoven webs of this invention may be used in filtering masks that areadapted to cover at least the nose and mouth of a wearer. Generally, acup-shaped body portion is adapted to fit over the mouth and nose of thewearer. A strap or harness system may be provided to support the mask onthe wearer's face. Although a single strap is common, a harness may beemployed with more than one strap and may come in a variety ofconfigurations—see, for example, U.S. Pat. No. 4,827,924 to Japuntich etal., U.S. Pat. No. 5,237,986 to Seppalla et al., and U.S. Pat. No.5,464,010 to Byram.

Examples of other filtering face masks where nonwoven fibrous electretwebs may be used include U.S. Pat. No. 4,536,440 to Berg, U.S. Pat. No.4,807,619 to Dyrud et al., U.S. Pat. No. 4,883,547 to Japuntich, andU.S. Pat. No. 5,374,458 to Burgio. As shown in these patents, thenonwoven fibrous electret web is used as a filter in the cub-shaped maskbody. The electret filter media also may be used, for example, in afilter cartridge for a respirator, such as the filter cartridgedisclosed in U.S. Pat. No. 5,062,421 to Burns and Reischel.

The applicant believes that the hydrocharging method without the use ofapplied electric voltages creates both positive and negative charge ontothe substrate, such that the positive and negative charge is randomlydispersed throughout the web. Random charge dispersal produces anunpolarized web. Thus, a nonwoven fibrous electret web produced inaccordance with this group of teachings of the present invention, may besubstantially unpolarized in a plane normal to the plane of the web.“Substantially unpolarized trapped charge” refers to a fibrous electretweb that exhibits less than 1 mu.C/m.sup.2 of detectable dischargecurrent using TSDC analysis, where the denominator is the electrodesurface area. This charge configuration can be shown by subjecting theweb to thermally-simulated discharge current (TSDC).

Thermally-stimulated discharge analysis is detailed in U.S. Pat. No.6,824,718 and its references, and they are incorporated here byreference.

Charge Neutralization and Control of Charge on Substrates

The control and elimination of substrate electrostatic charge is anotherbranch of electrostatic substrate processing. It differs from electretmaking in that here it is desired to remove electrical charge from asubstrate, instead of producing charges in a media or on a substrate.The known art of neutralizing charge on a web suffers from its inabilityto achieve near total neutralization.

For the purposes of the present invention, the following terms used inthis application are defined as follows:

-   -   “removal of electrical charge from a surface” means the        reduction of the force exerted by electrical charge on or in a        substrate on any electrically charged object near the surface;    -   “an electric field” is said to exist in any region of space in        which an electric charge would experience an electrical force;    -   “electric field intensity” means the vector force quantity        having the direction of force that would be exerted on a small        positive test charge placed at a point, and the magnitude is        equal to the force divided by the magnitude of the test charge;    -   “neutralization of electrical charge on a substrate” means the        reduction of the average electrical field intensity at or above        the substrate surface;    -   “web” means a sheet of material having a dimensional width in        one direction, and an indeterminate length in the orthogonal        direction, and a thickness in a third direction, which is        orthogonal to both the width and length;    -   “treatment fluid” means a fluid that facilitates the transfer of        electrical charge to or from the target substrate surface to or        from an electrically grounded conductive member.        Charge Neutralization with a Nonwetting Fluid

Substrate charge neutralization is carried out in the apparatusillustrated in FIG. 8. In this station 1000 the substrate is a film web1002 moving in the direction indicated by arrow 1008. Devices 1002 and1004 apply a conductive fluid to the web. A useful fluid is filtered tapwater with a minimum of dissolved solids. A useful fluid is one that isat least semiconductive with a conductivity greater than 500pico-Siemens per meter. It is inexpensive, non-hazardous and readilyavailable. More preferred is water with a minimum of dissolved gasesthat may form bubbles and a minimum of entrained air bubbles. Smallbubbles may attach themselves to flow surfaces and to the substrate andcause disturbances in the flow patterns and distribution of the water.Purity of the water required will be dictated by product or intermediatebeing processed.

Devices 1004 and 1002 may be any devices that apply a continuous film ofwater to the side of the substrate 1002 to which it abuts. The devicesare constructed so that an electrically conductive path exists from theapplicator surface contacting the water to three phase separation lines,where water is rejected from the surface. An electric conductor (notshown) is provided between the contacting applicator surface and anelectrical ground.

The preferred 1004 device is a fluid support and dispensing elementillustrated in FIG. 9.

Referring to FIG. 9, fluid is flowed to chamber 1107 and passes throughwall 1108 into the space 1109 between the nonporous web 1102 and member1103. It is preferred that 1103 be constructed from an electricallyconductive material and be grounded. Wire 1113 connects member 1103 toan electrical ground indicated symbolically by 1114. This provides ashort and direct path between the surface of 1102 and ground for theefficient movement of charged species. Water is supplied to space 1109at a rate sufficient to allow web 1102 to pass around member 1103without excessive friction. A rate sufficient to fully support web offthe applicator member and prevent touching is desired.

This rate will depend upon the tension and the speed of movement of web1102 and the exact geometry and flow distribution of water throughapplicator wall 1108.

When treating nonwetting webs, water will pond in the gap 1115 betweenthe member 1103 and the web. As additional water is brought to gap 1115,it will flow by gravity down the right hand side of 1103 as indicated byarrow 1111. A dynamic three phase contact line 1110 will be maintainedon the surface of web 1102. The surface of the water 1112 is indicatedin FIG. 9. At the contact line, water will be rejected from the surfaceof the web and be retained in the liquid pool in gap 1115. In this way,substantially dry web will leave the top of 1103. At the three phasecontact line, surface tension and gravity act to remove the water fromthe web while the viscosity acts to retard the separation.

Since the substrate is moving away from the contact line, the contactangle is a receding contact angle. It is preferred that this angle be aslarge as possible.

Theoretical analysis of the liquid flow shows an important parametercontrolling the separation of the water from the web at low speeds isthe Capillary Number defined as the product of the liquid viscosity,times the web speed, times the inverse of the liquid surface tension.Here low speed means speeds at which inertial effects do not dominatethe fluid flow. Furthermore, at low speeds, separation will be improvedif the Capillary Number is minimized so the dewetting surface tensionforces can dominate the process. This is easily done by reducing speedor more preferably, by increasing the treatment fluid temperature. It isfound raising the temperature when using water reduces the waterviscosity much more rapidly than the surface tension is diminished, thusit produces a net reduction in Capillary Number. Elevating thetemperature from 10 C to 80 C results in a significant reduction in theCapillary Number without changing web speed. This promotes drainage ofwater from the web. Free drainage from a contact line proceeds best ifthe dimensionless Capillary Number is below 0.02. Below 0.002 is mostpreferred.

The use of heated water is also advantageous in that it increases themobility of charged species to and from the web surface. The electricalconductivity of pure water is found to increase by a factor greater than100 over the range of 25 C to 100 C. Furthermore, heated water enhancesthe cleaning of and particulate removal from the web surface. Itprovides heat energy for more rapid removal of absorbed and adsorbedcontaminants. The heating can in some instances improve the mobility ofcharges imbedded within the web so as to improve the ability toneutralize them.

Heated water promotes cleaning of the substrate surfaces. Heated waterprovides energy for removal of the adsorbed and absorbed contaminants.

Particles attached to the surface have a high probability of trappingdrops of liquid on the surface when they leave the contact line. Theseshould be avoided. It is a teaching of this invention to cleancontaminant particles from the substrate surface prior to or during theprocess of charge removal.

With the geometry of FIG. 9, water is brought into intimate contact withboth a grounded member 1103 and the charge on the web 1102. This allowspositive and negative species to be exchanged between the web and thewater, and the water and ground. In this manner, charges on the websurface facing member 1103 may interact with the water and beneutralized. It is believed that negative charge is drained from the websurface by removal of electrons to the ground and positive charges areneutralized by the flow of electrons from the ground. Charges within theweb and on the opposite side are counterbalanced by charged speciesattracted to the wet side of the web. These come from the water creatinga net balance of zero when sensed from a distance. However, on the scaleof the dimensions of the web thickness, an electric field still existsat the surface of the web. For this reason, it is preferred tosimultaneously treat both sides of the web with devices providingcontact with water where the water contacts a grounded surface.

Tap water rather than pure triple distilled water is preferred. However,for some applications a high dissolved mineral content with some watersupplies will not be desirable. It is to be noted that even distilledand deionized water are conductive enough to function in this inventivecharge removal process. Other preferred fluids are polar fluids with aspecific gravity greater than 0.5. More preferred are these types ofpolar fluids that do form a three phase contact line when dewetting fromthe target substrate. Still other preferred fluids are those with aspecific gravity greater than 0.5 and are at least semiconductive withan electrical conductivity of greater than 500 picro-Siemens per meter.

Advantage in operation may be obtained by segmenting the fluid flow intotwo or more regions. FIG. 9 shows three internal chambers 1107, 1105 and1106 within member 1103. These are formed by internal partition 1116.Fluid may be flowed at different rates into each. Different fluidcompositions may be flowed into each chamber. The liquid which firstcontacts the web 1102 is that which flows from chamber 1107, and theinitial separation gap 1109 is set by the flow from 1107. The initialprimary contacting fluid maintains contact with the web until itapproaches the separation line 1110. An additional secondary fluid addedby chamber 1105 is introduced between the initial fluid and the wall1108. The initial contacting fluid may have a composition or purity thatdiffers from the secondary fluid. It is preferred that the primary fluidhave a higher purity than the secondary.

A preferred method of neutralizing and removing charge from a web at lowCapillary Number conditions is illustrated in FIG. 8. Here, devices 1004and 1002 are arranged in close proximity to each other. The web 1002passes between them in region 1010 where water is provided on both sidesof it. Preferably, there exists at least one plane where water contactssimultaneously both sides of the web and where that extends across theweb and is perpendicular to the web surface. It is preferred that bothdevices 1002 and 1004 be of the type of water dispensing devicesdescribed above and illustrated in FIG. 9.

Water flowing from the station 1000 will be caught in pan 1006 and maybe recycled. The viscous shear flow of liquid between the applicatorsand the web washes the surface. Stresses are exerted upon anyparticulates on the surfaces. Adsorb contaminant molecules diffuse intothe liquid thus cleaning the substrate. Since the washing action of theapplicators removes contaminants, purification and filtering ispreferred before reuse in this process.

When the water does not wet the web, the dewetting of the web at thethree phase contact line provides removal of the water without anevaporative drying step.

In air charged contaminating particles on the surface of a web are heldtenaciously by the attractive electrostatic forces at their contactpoints. By surrounding them with conductive water these charges areneutralized and the particles are more easily rermoved. The liquid flowshear stress existing in gap 1109 will act to dislodge such particles.Once entrained in the water, they will tend to be rejected with thewater from the film surface at the contact line 1110.

Other devices may be used to contact the substrate with water. Theseinclude, but are not limited to roll applicators, slot die applicators,curtain coaters, and jet coating applicators. The simple dip contactingdevice of FIG. 10 is an embodiment of this invention. Traditional dipcoating is described in Schunk, Hurd and Brinker in Chapter 13 of LiquidFilm Coating, ISBN 0412064812, editors Kistler and Schweizer, publishedby Chapman and Hall. In traditional dip coating the liquid wets thesubstrate. The liquid need not be electrically conductive and is oftenan organic solution. Grounding of the liquid is not required. The methodof this invention provides liquid free web leaving the applicatordevice. This is not like coating where the purpose is to provide aliquid covering on the web leaving the device.

FIG. 10 is a cross sectional view of a dip treatment station 1200 ofthis method. Web 1202 which is not wet by the treatment fluid 1216 isdirected into a vessel 1210. It enters the fluid surface 1208 through adisturbance suppression baffle 1214. This baffle surrounds the web onits sides and is open ended to allow free passage of the web and thefluid. Web leaves the station 1200 in a vertical direction indicated byarrow 1201.

The baffle prevents disturbances and entrained air bubbles created bythe entrance of the web into the fluid from disturbing the two threephase contact lines 1206 and 1218. The web, after entering the fluid, isredirected by roller 1212 to pass vertically out of the free surface1208 of the fluid 1216. The web locally distorts the free surface 1208as it exits the liquid pool. Three phase contact lines 1206 and 1218 areformed as the nonwetting fluid is rejected by the web. Electricallyconductive plate 1224 submerged in the fluid 1216, is connect by anelectrical conductor 1220 to an electrical ground 1222. The groundedfluid allows charge carrying species to pass to and from the web surfaceso that the surface of the web is discharged, and embedded chargedspecies within the web are neutralized.

It is also a teaching to flush air from the surfaces of substrates witha gas that is highly absorbable by the treatment liquid. The methods aredescribed in the electret processing, teaching of this application. Thisallows a substrate that is not wet by water or treatment fluid to becompletely covered with all surfaces treated by the water without air orgas remaining attached to the surfaces or within the media.

It is preferred that a means also be used to periodically orcontinuously provide clean water to the tank 1210. It is preferred thatthis liquid is provided in the vicinity of the substrate exiting fromthe vessel through porous conductive tubes 1232 and 1234. Water ispumped to these by a means not shown. These tubes are also electricallygrounded. When water is supplied it will overflow through opening 1220and exit down drain 1230. This overflow removes contaminates collectedon the surface of the water along with those in the bulk. All the bulkand surface water is thus replaced in time.

While FIG. 10 shows a device for the treatment of web, the device may beused to treat individual sheets of substrate when the roll and baffleare removed, and replaced by sheet handling means to submerge the sheetin the fluid and withdraw it from the fluid. Examples of sheets that maybe treated are plastic film sheets, glass plates, semiconductor wafers,and metal plates. One skilled in the art will recognize that thistreatment method is also useful for treatment of items other thansheets, such as blocks of a cylindrical shape, tubular shape,rectangular shape, and any shape that allows the formation of a fluidrejecting three phase contact line and the rejection of fluid from itssurface, or a portion of its surface by a three phase contact line.

FIG. 11 illustrates another method of removal of treatment liquid fromthe surface of the web. The free drainage of liquid is replaced with awiping apparatus 1400. Rubber wiping blades 1404 and 1406 are mounted infixtures 1402 and 1408. These are manipulated so as to bring the edgesof the wiping blades to bear against the surface of the web 1410 onopposite sides of the web at liquid removal point 1424. It is preferredthat the blades contact the web directly opposite each other. Theworking contact pressure must be determined by trial as it will dependupon web speed and surface along with treatment liquid properties.

The action of the blades blocks the upwards movement of the electricallygrounded treatment liquid 1412. They cause the liquid to form acontinuous flowing cascade 1416 back into the surface 1414 of the pondedliquid 1412. By this means, a continuous liquid path is maintainedbetween the removal point 1424 to the electrical grounded electrode1418. Electrode 1418 is connected by wire 1420 to an electrical ground1422. Surprisingly, the removal of the liquid by the wiping blades by arubbing wiping contact does not itself produce static charge on the web.The high shear of the rubbing contact is also efficacious in removingparticulate surface contaminants.

Urethane rubber squeegee material is preferred for the rubber wipingblades 1404 and 1406. These are known in the printing industry andwidely available from such suppliers as UV Process Supply, Inc. ofChicago, Ill., USA.

Apposing air doctors may be used as illustrated in FIG. 12 to remove thetreatment liquid. Air doctors 1504 and 1506 are positioned on oppositesides of the web 1510. These doctors are porous metal tubes into whichhigh pressure air is injected by a means not shown. They are positionedacross the width of the web with a small gap 1524 maintained betweenthem. Gaps of 0.2 to 2 millimeters greater than the web thickness arepreferred. Air issues from the porous wall of the tubes around theircircumference. In the constricted space of the small gap 1524 the airflow from the air doctors creates a high pressure zone. This pressureand the air flow from this zone sweeps the liquid 1512 from the surfacesof the web 1510. This action blocks the upwards movement of theelectrically grounded treatment liquid 1512 when the web moves pastthem. A continuous flowing cascade 1516 of liquid back into the surface1514 of the ponded liquid is formed. By this means a continuous liquidpath between the removal point in gap 1524 and the electrical electrode1518 is maintained. Electrode 1518 is connected by wire 1520 to anelectrical ground 1522.

Those skilled in the art will also recognize that other air nozzledevices may also be used to sweep the treatment liquid from the web.Nozzles designed according to the teachings of U.S. Pat. No. 2,135,406are useable.

It is also within the scope of this invention to perform other functionswithin the treatment fluid within the treatment station. These includebut are not limited to cleaning, heating, chemical adsorption, sonic orultrasonic treatment, or irradiation with electromagnetic radiation.Those skilled in the art will recognize that tank 1210 of FIG. 10 may bepartitioned to perform one or more of these functions. These additionalfunctions may also be performed in a separate apparatus prior toentering the neutralization process.

FIG. 1 of U.S. Pat. No. 4,363,070 illustrates a great failing of knowntechnology. The charge removal bristles of its teaching contact the web10 (of this FIG. 1) as it is supported on a contacting roller 16 (ofthis FIG. 1). As the web translates around this roller and leaves it,triboelectric charging takes place creating a new charge on the web. Therecharging of the web is further exacerbated by the additionalcontacting of the other side of the web by roller 18. FIG. 1 of U.S.Pat. No. 4,$831,488 also illustrates the same failing. The “removal”device rolls generate static charge on the web by the action ofseparating the web from their surfaces. Whitmore in U.S. Pat. No.3,671,806 teaches the same failing in his FIG. 1. Grarovsky in U.S. Pat.No. 5,394,293 illustrates the same failing in his FIG. 2. Carter, et al.in U.S. Pat. No. 5,041,941 illustrates the same failing in their FIG. 1.Chapman in U.S. Pat. No. 878,273 illustrates the same failing in hisdrawing.

Alternatively, sacrificial areas on the substrate may be provided forphysical touching while other critical areas are not touched. Web may betransported by touching only the edges. This known technology includesweb edged with sprocket holes and sprocket gear drive systems. Anexample is the transport of 35 millimeter photo film through processors.

Another inventive teaching of this application is to transport thesubstrate from the static removal station without contacting the surfacewith a solid object. Directional displacement, web transport andhandling are accomplished with fluid flotation devices. Useful fluidshave a density less than 0.5 grams per centiliter at 0 degrees Celsiusand 1 atmosphere pressure. Other useful fluids include a polar fluid, anaqueous fluid or a conductive fluid with a specific gravity greater than0.5, when used in conjunction with an ground. Contacting a web with afluid of a specific gravity greater than 0.5 may cause static chargingwhen the fluid is nonconductive. Additionally, the creation of drops ofliquid on the web surface, followed by the removal of the drops willleave behind electrical charges on the web. Therefore, any contactingdevice using fluids of a specific gravity greater than 0.5 is preferredif it is operated as a charge removal device as describe previously.

The physical touching of the substrate is nearly unavoidable whenmanufacturing articles. However, there are often very criticalintermediate steps that require very low or near zero surface charge fortheir execution. The method and apparatus of this invention allow theaccomplishment of crucial manufacturing steps with neutralized substratesurfaces.

Those skilled in the art of treatment will recognize that fluids otherthan water may be used for this treatment. These will be characterizedby forming a three phase contact line and having a preferred electricalconductivity equal to or greater than distilled water. Additionally,polar fluids or fluids that will absorb water from the atmosphere may beused if the conductance at the three phase contact line is greater than10000 pico-Siemens per meter and more preferably greater than 1micro-Siemens per meter.

Charge Neutralization with a Wetting Fluid

The method previously described must be modified if the treatment liquidwets the substrate, is absorbed by the substrate, or is entrained by thesubstrate. Here, removal of the substrate from a liquid surface orseparation from a liquid application device will carry with it entrainedliquid. No three phase contact line will form. FIG. 13 illustrates astation 1300 for removing electrical charge from a web which is wettedby the treatment liquid. Applicators 1304 and 1303 are liquidapplicators that apply conductive liquid to both sides of the web 1302.The liquid in or on these devices 1303 and 1304 is electricallyconnected to an electrical ground by means not shown. Additionally, thetreatment liquid is required to be volatile. Volatile means that a phasechange from liquid to gas can be forced to occur in the drying regionindicated by dashed line 1314.

Many different types of applicators may be used. The direction ofmovement of the web is indicated by arrow 1308. A portion of the liquidapplied to the web is retained as a liquid film coating, indicated byheavy line 1316, and is carried away from the applicators 1304 and 1303.The web and coating on it proceeds to a drying step which takes place inregion 1314. Here, the liquid is evaporated by a means not shown. Inorder to produce neutralization of the web surfaces leaving station1300, drying lines 1310 and 1311 are maintained on their respectivesides of web 1302. The drying line is where the volatile mass of liquidon the surface is exhausted. From near this point back to the liquidapplicator and to the liquid electrical grounding means, a conductivefluid path is provided. This allows exchange of charged species with theweb until the final molecular layers of liquid are removed.

It is has been found that the evaporation of water from a dielectric webmay leave behind surface charge. It is most intense when many smalldrops of nonwetting liquid are evaporated. When a wetting and conductiveliquid forms a continuous liquid film and is evaporated, the charging iszero or near zero. When small drops of a nonwetting liquid areevaporated, charging of the web may occur although it is not alwaysdetectable with conventional static meters. Since positive andnegatively charge areas may be created and interspersed, the net chargesensed by a field meter one inch distant from the surface is zero ornear zero.

It is preferred that the volatile liquid not change from wetting tononwetting during drying. This could occur when the volatile materialabsorbs a second volatile from the gas in the drier.

Static charge removal treatment with a wetting fluid is achieved in thefollowing manner. First, if the web is nonporous, applicators 1304 and1303 are used to apply equal coatings of the treatment fluid to bothsides of the web. These coatings are uniform across the width and lengthof the substrate. Uniformity is preferred in drying. Drying in region1314 is best carried out so as to produce a single curvilinear dryingline on each side of the substrate. This gives a continuous electricalconnection of the wetted surface from the drying lines to electricalground. Straight drying lines are most preferred.

It is undesirable to create surface areas on the substrate wetted byliquid that are not connected by liquid to ground. Such areas are likelyto contain unbalanced electrical charge. This will be left behind whenthis area is dried.

Pure wetting treatment liquids are preferred. It is found that mostpreferred of these are liquids that have surface tensions that decreasewith increasing temperature at the conditions of drying.

It is preferred to shield the drying substrate with conductive andgrounded electrodes during the drying process to avoid inductivecharging during drying.

If the wetting liquid is a mixture of chemical species, the morevolatile species when incrementally removed should leave behind amixture that is lower in surface tension.

If the web is porous and is wet by the treatment fluid, then it ispreferred to apply a uniform fluid coating to one side of the substrate.

The wiping and sweeping devices of FIGS. 11 and 12 may be used with bothwetting and nonwetting liquids. In the case of wetting liquids, thesedevices act as coating devices that apply thin coatings to the web withminimized drying requirements. Other methods of applying thin coatingsof conductive, wetting liquids are also useful for static chargeelimination.

One of the simplest methods for applying wetting liquid to both sides ofa film web while maintaining an electrical path from the drying line toa ground is illustrated in FIG. 14. Web 1602 enters the charge removalstation 1600 and encounters a train of contacting rolls 1603. Wettingtreatment liquid is intermittently placed on both sides of the web oronto the surface of the first two rolls of the train. The roll trainspreads the liquid into a coating on each side of the web that isuniform in the down web direction, but not necessarily in the cross webdirection as the web leaves rolls 1604 and 1605. The apparatus andmethods are described in detail by Leonard et al. in U.S. Pat. No.6,737,113 and U.S. Pat. No. 6,878,408, the contents of which areincluded here by reference.

The roll surfaces of 1604 and 1605 are electrically conductive andconnected to an electrical ground. The web then proceeds from theserolls with a conductive layer of liquid which is electrically connectedto a ground. From the roll train the web passes untouched to a drierzone 1614 represented by dashed lines.

A portion of the liquid applied to the web is retained as a liquid filmcoating indicated by heavy line 1616. It is on both sides of the web andis carried away from the applicators 1604 and 1605. The web and coating1616 on it proceeds to a drying step which takes place in a regiondefined by dashed box 1614. Here, the liquid is evaporated by a meansnot shown. In order to produce neutralization of the web surfacesleaving station 1600, drying lines 1610 and 1611 are maintained on theirrespective sides of web 1602. The drying line is where the volatile massof liquid is exhausted. From this point back to the liquid applicatorand to the electrically grounded rolls 1604 and 1605, a conductive fluidpath is provided. This allows exchange of charged species with the webuntil the final molecular layers of liquid are evaporated.

Charge Neutralization with a Porous Substrate

The previously described method and apparatus need modification for thetreatment of porous or nonwoven webs.

The hydrocharging of nonwoven substrates has been taught in a previoussection of this application. The application of nonwetting liquid to anonwoven will generally create hydrocharging. The most effective chargeremoval from a porous web uses treatment with a wetting fluid and usesof the procedures described above. It is preferred that the treatmentliquid remain a wetting liquid throughout the drying process.

Those of ordinary skill in the art will know, or be able to ascertain,using no more than routine experimentation or process simulation, manyequivalents to the specific embodiments of this invention as describedherein. These and all other equivalents and combinations of theembodiments are intended to be encompassed by the claims. Allpublications and references cited herein, including those in thebackground section, are expressly incorporated by reference in theirentirety.

The embodiments above are chosen, described and illustrated so thatpersons skilled in the art will be able to understand the invention andthe manner and process of making and using it. The descriptions and theaccompanying drawings should be interpreted in the illustrative and notthe exhaustive or limited sense. The invention is not intended to belimited to the exact forms disclosed. While the application attempts todisclose all of the embodiments of the invention that are reasonablyforeseeable, there may be unforeseeable insubstantial modifications thatremain as equivalents. It should be understood by persons skilled in theart that there may be other embodiments than those disclosed which fallwithin the scope of the invention as defined by the claims. Where aclaim, if any, is expressed as a means or step for performing aspecified function, it is intended that such claim be construed to coverthe corresponding structure, material, or acts described in thespecification and equivalents thereof, including both structuralequivalents and equivalent structures, material-based equivalents andequivalent materials, and act-based equivalents and equivalent acts.

1. A method of making an electret, comprising the steps of: a. providinga substrate; b. removing a first gas from the substrate with afunctional, second, gas; c. adding at least one functional liquid to thesubstrate; and d. removing the functional liquid from the substrate. 2.The method of making an electret of claim 1, whereby the step ofremoving the first gas involves the functional gas displacingsubstantially all of the first gas from the substrate.
 3. The method ofmaking an electret of claim 1, whereby the step of adding a functionalliquid replaces substantially all of the functional gas from thesubstrate and creates a substantially continuous, covering layer offunctional liquid on the substrate.
 4. The method of making an electretof claim 1, whereby the step of removing the functional liquid breaksdown the functional liquid into a distribution of drops and subsequentlycompletely removes the drops.
 5. The method of making an electret ofclaim 1, wherein the substrate is an element selected from the group ofelements consisting of a sheet, a piece part, an article, a web, freeparticles, and free fibers.
 6. The method of making an electret of claim1, wherein the substrate is selected from the group consisting ofpolymeric porous and non porous materials.
 7. The method of making andelectret of claim 1, wherein the first gas is air.
 8. The method ofmaking an electret of claim 1, wherein the step of removing the firstgas involves flushing by the functional gas.
 9. The method of making anelectret of claim 1, wherein the step of removing the first gas includesapplying a functional gas to the substrate via a contactor.
 10. Themethod of making an electret of claim 1, further comprising the step ofheating the functional gas.
 11. The method of making an electret ofclaim 1, wherein the functional gas is steam.
 12. The method of makingan electret of claim 1, wherein the functional gas is condensed to aliquid.
 13. The method of making an electret of claim 1, wherein thestep of adding a functional liquid involves immersing the substrate inthe functional liquid.
 14. The method of making an electret of claim 1,wherein the step of adding a functional liquid involves applying thefunctional liquid to the substrate via a contactor.
 15. The method ofmaking an electret of claim 1, wherein the step of adding a functionalliquid involves condensing a heated functional gas on to the substrate.16. The method of making an electret of claim 1, wherein the functionalliquid has a conductivity of equal to or greater than 500 pico-Siemensper meter.
 17. The method of making an electret of claim 16, wherein thefunctional liquid is a degassed liquid.
 18. The method of making anelectret of claim 16, wherein the functional liquid is water.
 19. Themethod of making an electret of claim 1, further comprising the step ofheating the functional liquid.
 20. The method of making an electret ofclaim 1, wherein the step of removing the functional liquid is at leastone mechanical step selected from the group consisting of blowing,centrifuging, sucking, vacuuming, gravity draining, vibrating, scraping,and squeezing.
 21. The method of making an electret of claim 1, whereinthe step of removing the functional liquid is at least one phase changestep.
 22. The method of making an electret of claim 1, wherein the stepof removing functional liquid involves first mechanically removing aportion of the functional liquid from the substrate, and secondlyremoving any remaining portion of the functional liquid from thesubstrate via a phase change.
 23. The method of making an electret ofclaim 1, wherein the steps of removing the first gas and adding afunctional liquid are combined into one step involving exposing thesubstrate to a 2-phase functional gas and functional liquid mixture. 24.The method of making an electret of claim 23, wherein the exposing stepis repeated.
 25. The method of making an electret of claim 23,comprising a step of adding an additional functional liquid to thesubstrate after the exposing step.
 26. The method of making an electretof claim 23, wherein the 2-phase functional gas and functional liquidmixture is boiling water.
 27. The method of making an electret of claim1, at least one of the adding and removing steps are repeated.
 28. Themethod of making an electret of claim 1, further comprising the step ofcleaning the provided substrate.
 29. The method of making an electret ofclaim 28, wherein the cleaning step is accomplished before the step ofremoving the first gas.
 30. The method of making an electret of claim28, wherein the cleaning step is accomplished during the step ofremoving the first gas.
 31. The method of making an electret of claim28, wherein the cleaning step is accomplished after the step of removingthe first gas.
 32. The method of making an electret of claim 1, whereinthe cleaning step involves washing the substrate with a wetting liquid.33. The method of making an electret of claim 28, wherein the cleaningstep is accomplished by heating the functional gas or functional liquid.34. The method of making an electret of claim 1, further comprising thestep of cleaning the functional gas or functional liquid.
 35. The methodof making an electret of claim 1, further comprising the step ofabsorbing, adsorbing, reacting, dissolving, or condensing the functionalgas after the step of removing the first gas and prior to the step ofadding a functional liquid.
 36. The method of making an electret ofclaim 1, further comprising the step of absorbing, adsorbing, reacting,dissolving, or condensing the functional gas after the step of removingthe first gas and simultaneous with the step of adding a functionalliquid.
 37. The method of making an electret of claim 1, furthercomprising the step of adding at least one charge additive to thefunctional liquid.
 38. The method of making an electret of claim 37,wherein the charge additive is at least one compound selected from thegroup consisting of fluorinated oxazolidinones, fluorinated piperazines,and perfluorinated alkanes.
 39. The method of making an electret ofclaim 1, further comprising the step of applying an electrical charge tothe functional liquid.
 40. The method of making an electret of claim 1,further comprising the step of appending additional elements to thesubstrate, the elements being selected from the group consisting ofparticles, molecules, colloids, and aggregations.
 41. The method ofmaking an electret of claim 40, wherein the step of appending elementsto the substrate is accomplished after the functional liquid removalstep.
 42. The method of making an electret of claim 39, wherein thesubstrate is a polymer selected from the group consisting of polyolefin,polyester, polycarbonate, polystyrene, polyphenylenesulfate,fluorine-based resin, parylene and mixtures thereof.
 43. The method ofmaking an electret of claim 1, wherein the electret surface propertiesare targeted to a predetermined filtration challenge.
 44. The method ofmaking an electret of claim 43, wherein the filtration challenge is aliquid mist in a gas or a liquid drop in an immiscible liquid.
 45. Themethod of making an electret of claim 44, wherein the targeting involvesadjusting the combination of substrate surface and the gas properties toprevent wetting of the substrate by the mist.
 46. The method of makingan electret of claim 44, wherein the targeting involves adjusting thecombination of substrate surface properties and the gas conditions toprevent wetting of the substrate by the mist.
 47. The method of makingan electret of claim 44, wherein the adjusting involves adjusting thecontact angle of the liquid of the mist to greater than 45 degrees bylowering the surface energy of the substrate in the gas.
 48. The methodof making an electret of claim 45, wherein the adjusting involvesadjusting the contact angle to greater than 45 degrees by increasing thesurface tension of a contaminant mist in the gas.
 49. A method of makingan electret, comprising the steps of: a. providing a polymeric substrateselected from the group consisting of a sheet, a piece part, an article,free fibers, and webs; and b. hydrocharging the substrate by: i.removing air from the substrate with a functional gas, whereby the gasdisplaces substantially all of the air from the substrate; ii. adding atleast one functional liquid to the substrate, whereby the functionalliquid replaces substantially all of the functional gas from thesubstrate and creates a substantially continuous, covering layer offunctional liquid on the substrate; and iii. removing the functionalliquid from the substrate, whereby the functional liquid breaks downinto a distribution of drops on the substrate and subsequently areremoved.
 50. A method of making an electret, comprising the steps of: a.providing a substrate selected from the group consisting of a sheet, apiece part, an article, free particles, free fibers, and webs; and b.hydrocharging the substrate by: i. removing air from the substrate byimmersing the substrate in boiling water; ii. removing the substratefrom the boiling water; and iii. removing at least some portion waterfrom the substrate by drying.
 51. A method of making an electretcomprising the steps of providing a substrate in a noncondensible gas;replacing the noncondensible gas with a functional liquid; selecting thefunctional liquid or the operating conditions so that the noncondensiblegas is functional with respect to the functional liquid; and removingthe functional liquid.
 52. A method of controlling charge on asubstrate, comprising the steps of: a. moving a substrate; b. applyingat least one grounded liquid to at least one side of the movingsubstrate; c. removing the liquid from the substrate; and d. maintainingan electrically conductive path between the substrate and the liquiduntil it is removed.
 53. The method of controlling charge on a substrateof claim 52, wherein some portion of the charge is neutralized.
 54. Themethod of controlling charge on a substrate of claim 52, wherein thesubstrate is an element selected from the group of elements consistingof a sheet, a piece part, an article, a web, free particles, and freefibers.
 55. The method of controlling charge on a substrate of claim 52,wherein the substrate is selected from the group consisting of porousand non porous substrates.
 56. The method of controlling charge on asubstrate of claim 52, wherein the liquid is non-wetting and forms athree phase contact line with respect to the substrate.
 57. The methodof controlling charge on a substrate of claim 56, wherein the substratemoves through a 3-phase contact line.
 58. The method of controllingcharge on a substrate of claim 52, wherein the removing step takes placeat a zone of removal, and wherein the electrically conductive path ismaintained from this zone to a liquid grounding electrode.
 59. Themethod of controlling charge on a substrate of claim 58, wherein theliquid is wetting and the zone of removal is a drying zone.
 60. Themethod of controlling charge on a substrate of claim 52, wherein theapplying step includes dipping.
 61. The method of controlling charge ona substrate of claim 52, wherein the applying step is implemented bymoving the substrate past at least one contactor applicator.
 62. Themethod of controlling charge on a substrate of claim 52, wherein thestep of adding a liquid to a first side of the moving substrate isimplemented by adding a liquid at a first segment, moving the substrateand adding a liquid at a second segment.
 63. The method of controllingcharge on a substrate of claim 52, further comprising the step of addingat least one liquid to a second side of the substrate by a secondgrounded contactor applicator.
 64. The method of controlling charge on asubstrate of claim 63, wherein the step of adding a liquid to the secondside of the substrate is implemented substantially simultaneously withthe step of adding a liquid to the first side of the web substrate. 65.The method of controlling charge on a substrate of claim 52, wherein thestep of removing includes wiping.
 66. The method of controlling chargeon a substrate of claim 52, wherein the step of removing includesevaporation.
 67. The method of controlling charge on a substrate ofclaim 52, wherein the step of removing includes blowing.
 68. The methodof controlling charge on a substrate of claim 52, wherein the step ofremoving includes at least one mechanical step selected from the groupconsisting of blowing, centrifuging, sucking, vacuuming, gravitydraining, vibrating, scraping, and squeezing.